diff --git a/src/Language/Lexer.php b/src/Language/Lexer.php
index 121016d..6dc975d 100644
--- a/src/Language/Lexer.php
+++ b/src/Language/Lexer.php
@@ -10,6 +10,8 @@ use GraphQL\Utils;
  * source lexes, the final Token emitted by the lexer will be of kind
  * EOF, after which the lexer will repeatedly return the same EOF token
  * whenever called.
+ *
+ * Algorithm is O(N) both on memory and time
  */
 class Lexer
 {
@@ -51,6 +53,26 @@ class Lexer
      */
     public $lineStart;
 
+    /**
+     * Current cursor position for UTF8 encoding of the source
+     *
+     * @var int
+     */
+    private $position;
+
+    /**
+     * Current cursor position for ASCII representation of the source
+     *
+     * @var int
+     */
+    private $byteStreamPosition;
+
+    /**
+     * Lexer constructor.
+     *
+     * @param Source $source
+     * @param array $options
+     */
     public function __construct(Source $source, array $options = [])
     {
         $startOfFileToken = new Token(Token::SOF, 0, 0, 0, 0, null);
@@ -61,6 +83,7 @@ class Lexer
         $this->token = $startOfFileToken;
         $this->line = 1;
         $this->lineStart = 0;
+        $this->position = $this->byteStreamPosition = 0;
     }
 
     /**
@@ -95,10 +118,11 @@ class Lexer
      */
     private function readToken(Token $prev)
     {
-        $body = $this->source->body;
         $bodyLength = $this->source->length;
 
-        $position = $this->positionAfterWhitespace($prev->end);
+        $this->positionAfterWhitespace();
+        $position = $this->position;
+
         $line = $this->line;
         $col = 1 + $position - $this->lineStart;
 
@@ -106,7 +130,8 @@ class Lexer
             return new Token(Token::EOF, $bodyLength, $bodyLength, $line, $col, $prev);
         }
 
-        $code = Utils::charCodeAt($body, $position);
+        // Read next char and advance string cursor:
+        list (, $code, $bytes) = $this->readChar(true);
 
         // SourceCharacter
         if ($code < 0x0020 && $code !== 0x0009 && $code !== 0x000A && $code !== 0x000D) {
@@ -121,7 +146,8 @@ class Lexer
             case 33: // !
                 return new Token(Token::BANG, $position, $position + 1, $line, $col, $prev);
             case 35: // #
-                return $this->readComment($position, $line, $col, $prev);
+                $this->moveStringCursor(-1, -1 * $bytes);
+                return $this->readComment($line, $col, $prev);
             case 36: // $
                 return new Token(Token::DOLLAR, $position, $position + 1, $line, $col, $prev);
             case 40: // (
@@ -129,8 +155,10 @@ class Lexer
             case 41: // )
                 return new Token(Token::PAREN_R, $position, $position + 1, $line, $col, $prev);
             case 46: // .
-                if (Utils::charCodeAt($body, $position+1) === 46 &&
-                    Utils::charCodeAt($body, $position+2) === 46) {
+                list (, $charCode1) = $this->readChar(true);
+                list (, $charCode2) = $this->readChar(true);
+
+                if ($charCode1 === 46 && $charCode2 === 46) {
                     return new Token(Token::SPREAD, $position, $position + 3, $line, $col, $prev);
                 }
                 break;
@@ -162,21 +190,24 @@ class Lexer
             case 105: case 106: case 107: case 108: case 109: case 110: case 111:
             case 112: case 113: case 114: case 115: case 116: case 117: case 118:
             case 119: case 120: case 121: case 122:
-                return $this->readName($position, $line, $col, $prev);
+                return $this->moveStringCursor(-1, -1 * $bytes)
+                    ->readName($line, $col, $prev);
             // -
             case 45:
             // 0-9
             case 48: case 49: case 50: case 51: case 52:
             case 53: case 54: case 55: case 56: case 57:
-                return $this->readNumber($position, $code, $line, $col, $prev);
+                return $this->moveStringCursor(-1, -1 * $bytes)
+                    ->readNumber($line, $col, $prev);
             // "
             case 34:
-                return $this->readString($position, $line, $col, $prev);
+                return $this->moveStringCursor(-1, -1 * $bytes)
+                    ->readString($line, $col, $prev);
         }
 
         $errMessage = $code === 39
-                    ? "Unexpected single quote character ('), did you mean to use ". 'a double quote (")?'
-                    : 'Cannot parse the unexpected character ' . Utils::printCharCode($code) . '.';
+            ? "Unexpected single quote character ('), did you mean to use ". 'a double quote (")?'
+            : 'Cannot parse the unexpected character ' . Utils::printCharCode($code) . '.';
 
         throw new SyntaxError(
             $this->source,
@@ -190,38 +221,34 @@ class Lexer
      *
      * [_A-Za-z][_0-9A-Za-z]*
      *
-     * @param int $position
      * @param int $line
      * @param int $col
      * @param Token $prev
      * @return Token
      */
-    private function readName($position, $line, $col, Token $prev)
+    private function readName($line, $col, Token $prev)
     {
-        $body = $this->source->body;
-        $bodyLength = $this->source->length;
-        $end = $position + 1;
+        $value = '';
+        $start = $this->position;
+        list ($char, $code) = $this->readChar();
 
-        while (
-            $end !== $bodyLength &&
-            ($code = Utils::charCodeAt($body, $end)) &&
-            (
-                $code === 95 || // _
-                $code >= 48 && $code <= 57 || // 0-9
-                $code >= 65 && $code <= 90 || // A-Z
-                $code >= 97 && $code <= 122 // a-z
-            )
-        ) {
-            ++$end;
+        while ($code && (
+            $code === 95 || // _
+            $code >= 48 && $code <= 57 || // 0-9
+            $code >= 65 && $code <= 90 || // A-Z
+            $code >= 97 && $code <= 122 // a-z
+        )) {
+            $value .= $char;
+            list ($char, $code) = $this->moveStringCursor(1, 1)->readChar();
         }
         return new Token(
             Token::NAME,
-            $position,
-            $end,
+            $start,
+            $this->position,
             $line,
             $col,
             $prev,
-            mb_substr($body, $position, $end - $position, 'UTF-8')
+            $value
         );
     }
 
@@ -232,126 +259,130 @@ class Lexer
      * Int:   -?(0|[1-9][0-9]*)
      * Float: -?(0|[1-9][0-9]*)(\.[0-9]+)?((E|e)(+|-)?[0-9]+)?
      *
-     * @param int $start
-     * @param string $firstCode
      * @param int $line
      * @param int $col
      * @param Token $prev
      * @return Token
      * @throws SyntaxError
      */
-    private function readNumber($start, $firstCode, $line, $col, Token $prev)
+    private function readNumber($line, $col, Token $prev)
     {
-        $code = $firstCode;
-        $body = $this->source->body;
-        $position = $start;
+        $value = '';
+        $start = $this->position;
+        list ($char, $code) = $this->readChar();
+
         $isFloat = false;
 
         if ($code === 45) { // -
-            $code = Utils::charCodeAt($body, ++$position);
+            $value .= $char;
+            list ($char, $code) = $this->moveStringCursor(1, 1)->readChar();
         }
 
         // guard against leading zero's
         if ($code === 48) { // 0
-            $code = Utils::charCodeAt($body, ++$position);
+            $value .= $char;
+            list ($char, $code) = $this->moveStringCursor(1, 1)->readChar();
 
             if ($code >= 48 && $code <= 57) {
-                throw new SyntaxError($this->source, $position, "Invalid number, unexpected digit after 0: " . Utils::printCharCode($code));
+                throw new SyntaxError($this->source, $this->position, "Invalid number, unexpected digit after 0: " . Utils::printCharCode($code));
             }
         } else {
-            $position = $this->readDigits($position, $code);
-            $code = Utils::charCodeAt($body, $position);
+            $value .= $this->readDigits();
+            list ($char, $code) = $this->readChar();
         }
 
         if ($code === 46) { // .
             $isFloat = true;
+            $this->moveStringCursor(1, 1);
 
-            $code = Utils::charCodeAt($body, ++$position);
-            $position = $this->readDigits($position, $code);
-            $code = Utils::charCodeAt($body, $position);
+            $value .= $char;
+            $value .= $this->readDigits();
+            list ($char, $code) = $this->readChar();
         }
 
         if ($code === 69 || $code === 101) { // E e
             $isFloat = true;
-            $code = Utils::charCodeAt($body, ++$position);
+            $value .= $char;
+            list ($char, $code) = $this->moveStringCursor(1, 1)->readChar();
 
             if ($code === 43 || $code === 45) { // + -
-                $code = Utils::charCodeAt($body, ++$position);
+                $value .= $char;
+                $this->moveStringCursor(1, 1);
             }
-            $position = $this->readDigits($position, $code);
+            $value .= $this->readDigits();
         }
 
         return new Token(
             $isFloat ? Token::FLOAT : Token::INT,
             $start,
-            $position,
+            $this->position,
             $line,
             $col,
             $prev,
-            mb_substr($body, $start, $position - $start, 'UTF-8')
+            $value
         );
     }
 
     /**
-     * Returns the new position in the source after reading digits.
+     * Returns string with all digits + changes current string cursor position to point to the first char after digits
      */
-    private function readDigits($start, $firstCode)
+    private function readDigits()
     {
-        $body = $this->source->body;
-        $position = $start;
-        $code = $firstCode;
+        list ($char, $code) = $this->readChar();
 
         if ($code >= 48 && $code <= 57) { // 0 - 9
+            $value = '';
+
             do {
-                $code = Utils::charCodeAt($body, ++$position);
+                $value .= $char;
+                list ($char, $code) = $this->moveStringCursor(1, 1)->readChar();
             } while ($code >= 48 && $code <= 57); // 0 - 9
 
-            return $position;
+            return $value;
         }
 
-        if ($position > $this->source->length - 1) {
+        if ($this->position > $this->source->length - 1) {
             $code = null;
         }
 
         throw new SyntaxError(
             $this->source,
-            $position,
+            $this->position,
             'Invalid number, expected digit but got: ' . Utils::printCharCode($code)
         );
     }
 
     /**
-     * @param int $start
      * @param int $line
      * @param int $col
      * @param Token $prev
      * @return Token
      * @throws SyntaxError
      */
-    private function readString($start, $line, $col, Token $prev)
+    private function readString($line, $col, Token $prev)
     {
-        $body = $this->source->body;
-        $bodyLength = $this->source->length;
+        $start = $this->position;
 
-        $position = $start + 1;
-        $chunkStart = $position;
-        $code = null;
+        // Skip leading quote and read first string char:
+        list ($char, $code, $bytes) = $this->moveStringCursor(1, 1)->readChar();
+
+        $chunk = '';
         $value = '';
 
         while (
-            $position < $bodyLength &&
-            ($code = Utils::charCodeAt($body, $position)) &&
+            $code &&
             // not LineTerminator
-            $code !== 0x000A && $code !== 0x000D &&
+            $code !== 10 && $code !== 13 &&
             // not Quote (")
             $code !== 34
         ) {
-            $this->assertValidStringCharacterCode($code, $position);
+            $this->assertValidStringCharacterCode($code, $this->position);
+            $this->moveStringCursor(1, $bytes);
 
-            ++$position;
             if ($code === 92) { // \
-                $value .= mb_substr($body, $chunkStart, $position - 1 - $chunkStart, 'UTF-8');
-                $code = Utils::charCodeAt($body, $position);
+                $value .= $chunk;
+                list (, $code) = $this->readChar(true);
+
                 switch ($code) {
                     case 34: $value .= '"'; break;
                     case 47: $value .= '/'; break;
@@ -362,45 +393,51 @@ class Lexer
                     case 114: $value .= "\r"; break;
                     case 116: $value .= "\t"; break;
                     case 117:
-                        $hex = mb_substr($body, $position + 1, 4, 'UTF-8');
+                        $position = $this->position;
+                        list ($hex) = $this->readChars(4, true);
                         if (!preg_match('/[0-9a-fA-F]{4}/', $hex)) {
                             throw new SyntaxError(
                                 $this->source,
-                                $position,
+                                $position - 1,
                                 'Invalid character escape sequence: \\u' . $hex
                             );
                         }
                         $code = hexdec($hex);
-                        $this->assertValidStringCharacterCode($code, $position - 1);
+                        $this->assertValidStringCharacterCode($code, $position - 2);
                         $value .= Utils::chr($code);
-                        $position += 4;
                         break;
                     default:
                         throw new SyntaxError(
                             $this->source,
-                            $position,
+                            $this->position - 1,
                             'Invalid character escape sequence: \\' . Utils::chr($code)
                         );
                 }
-                ++$position;
-                $chunkStart = $position;
+                $chunk = '';
+            } else {
+                $chunk .= $char;
             }
+
+            list ($char, $code, $bytes) = $this->readChar();
         }
 
         if ($code !== 34) {
             throw new SyntaxError(
                 $this->source,
-                $position,
+                $this->position,
                 'Unterminated string.'
             );
         }
 
-        $value .= mb_substr($body, $chunkStart, $position - $chunkStart, 'UTF-8');
+        $value .= $chunk;
+
+        // Skip trailing quote:
+        $this->moveStringCursor(1, 1);
 
         return new Token(
             Token::STRING,
             $start,
-            $position + 1,
+            $this->position,
             $line,
             $col,
             $prev,
@@ -422,43 +459,33 @@ class Lexer
 
     /**
      * Reads from body starting at startPosition until it finds a non-whitespace
-     * or commented character, then returns the position of that character for
-     * lexing.
-     *
-     * @param $startPosition
-     * @return int
+     * or commented character, then places cursor to the position of that character.
      */
-    private function positionAfterWhitespace($startPosition)
+    private function positionAfterWhitespace()
     {
-        $body = $this->source->body;
-        $bodyLength = $this->source->length;
-        $position = $startPosition;
-
-        while ($position < $bodyLength) {
-            $code = Utils::charCodeAt($body, $position);
+        while ($this->position < $this->source->length) {
+            list(, $code, $bytes) = $this->readChar();
 
             // Skip whitespace
             // tab | space | comma | BOM
             if ($code === 9 || $code === 32 || $code === 44 || $code === 0xFEFF) {
-                $position++;
+                $this->moveStringCursor(1, $bytes);
             } else if ($code === 10) { // new line
-                $position++;
+                $this->moveStringCursor(1, $bytes);
                 $this->line++;
-                $this->lineStart = $position;
+                $this->lineStart = $this->position;
             } else if ($code === 13) { // carriage return
-                if (Utils::charCodeAt($body, $position + 1) === 10) {
-                    $position += 2;
-                } else {
-                    $position ++;
+                list(, $nextCode, $nextBytes) = $this->moveStringCursor(1, $bytes)->readChar();
+
+                if ($nextCode === 10) { // lf after cr
+                    $this->moveStringCursor(1, $nextBytes);
                 }
                 $this->line++;
-                $this->lineStart = $position;
+                $this->lineStart = $this->position;
             } else {
                 break;
             }
         }
-
-        return $position;
     }
 
     /**
@@ -466,21 +493,22 @@ class Lexer
      *
      * #[\u0009\u0020-\uFFFF]*
      *
-     * @param $start
      * @param $line
      * @param $col
      * @param Token $prev
      * @return Token
      */
-    private function readComment($start, $line, $col, Token $prev)
+    private function readComment($line, $col, Token $prev)
     {
-        $body = $this->source->body;
-        $position = $start;
+        $start = $this->position;
+        $value = '';
+        $bytes = 1;
 
         do {
-            $code = Utils::charCodeAt($body, ++$position);
+            list ($char, $code, $bytes) = $this->moveStringCursor(1, $bytes)->readChar();
+            $value .= $char;
         } while (
-            $code !== null &&
+            $code &&
             // SourceCharacter but not LineTerminator
             ($code > 0x001F || $code === 0x0009)
         );
@@ -488,11 +516,97 @@ class Lexer
         return new Token(
             Token::COMMENT,
             $start,
-            $position,
+            $this->position,
             $line,
             $col,
             $prev,
-            mb_substr($body, $start + 1, $position - $start, 'UTF-8')
+            $value
         );
     }
+
+    /**
+     * Reads next UTF8Character from the byte stream, starting from $byteStreamPosition.
+     *
+     * @param bool $advance
+     * @param int $byteStreamPosition
+     * @return array
+     */
+    private function readChar($advance = false, $byteStreamPosition = null)
+    {
+        if ($byteStreamPosition === null) {
+            $byteStreamPosition = $this->byteStreamPosition;
+        }
+
+        $code = 0;
+        $utf8char = '';
+        $bytes = 0;
+        $positionOffset = 0;
+
+        if (isset($this->source->body[$byteStreamPosition])) {
+            $ord = ord($this->source->body[$byteStreamPosition]);
+
+            if ($ord < 128) {
+                $bytes = 1;
+            } else if ($ord < 224) {
+                $bytes = 2;
+            } elseif ($ord < 240) {
+                $bytes = 3;
+            } else {
+                $bytes = 4;
+            }
+
+            $utf8char = '';
+            for ($pos = $byteStreamPosition; $pos < $byteStreamPosition + $bytes; $pos++) {
+                $utf8char .= $this->source->body[$pos];
+            }
+            $positionOffset = 1;
+            $code = $bytes === 1 ? $ord : Utils::ord($utf8char);
+        }
+
+        if ($advance) {
+            $this->moveStringCursor($positionOffset, $bytes);
+        }
+
+        return [$utf8char, $code, $bytes];
+    }
+
+    /**
+     * Reads next $numberOfChars UTF8 characters from the byte stream, starting from $byteStreamPosition.
+     *
+     * @param $numberOfChars
+     * @param bool $advance
+     * @param null $byteStreamPosition
+     * @return array
+     */
+    private function readChars($numberOfChars, $advance = false, $byteStreamPosition = null)
+    {
+        $result = '';
+        $totalBytes = 0;
+        $byteOffset = $byteStreamPosition ?: $this->byteStreamPosition;
+
+        for ($i = 0; $i < $numberOfChars; $i++) {
+            list ($char, $code, $bytes) = $this->readChar(false, $byteOffset);
+            $totalBytes += $bytes;
+            $byteOffset += $bytes;
+            $result .= $char;
+        }
+        if ($advance) {
+            $this->moveStringCursor($numberOfChars, $totalBytes);
+        }
+        return [$result, $totalBytes];
+    }
+
+    /**
+     * Moves internal string cursor position
+     *
+     * @param $positionOffset
+     * @param $byteStreamOffset
+     * @return $this
+     */
+    private function moveStringCursor($positionOffset, $byteStreamOffset)
+    {
+        $this->position += $positionOffset;
+        $this->byteStreamPosition += $byteStreamOffset;
+        return $this;
+    }
 }
diff --git a/src/Utils.php b/src/Utils.php
index 1507182..6a41f57 100644
--- a/src/Utils.php
+++ b/src/Utils.php
@@ -290,15 +290,17 @@ class Utils
      */
     public static function ord($char, $encoding = 'UTF-8')
     {
+        if (!$char && '0' !== $char) {
+            return 0;
+        }
         if (!isset($char[1])) {
             return ord($char);
         }
-        if ($encoding === 'UCS-4BE') {
-            list(, $ord) = (strlen($char) === 4) ? unpack('N', $char) : unpack('n', $char);
-            return $ord;
-        } else {
-            return self::ord(mb_convert_encoding($char, 'UCS-4BE', $encoding), 'UCS-4BE');
+        if ($encoding !== 'UCS-4BE') {
+            $char = mb_convert_encoding($char, 'UCS-4BE', $encoding);
         }
+        list(, $ord) = unpack('N', $char);
+        return $ord;
     }
 
     /**