Speed of mb_strlen varies a lot according to specified character set.
If you need length of string in bytes (strlen cannot be trusted anymore because of mbstring.func_overload) you should use <?php mb_strlen($string, '8bit'); ?>.
It's the fastest way (still a way slower than strlen, though) to determine byte length of string. Other single byte character sets (ASCII, ISO-8859-1, ...) are several times slower than 8bit.