mb_str_replace()... is slow. any alternatives?

As encoding is a real challenge when there are inputs from everywhere (utf8 or others), I prefer using only multibyte-safe functions. For str_replace, I am using this one which is fast enough.

if (!function_exists('mb_str_replace'))
{
   function mb_str_replace($search, $replace, $subject, &$count = 0)
   {
      if (!is_array($subject))
      {
         $searches = is_array($search) ? array_values($search) : array($search);
         $replacements = is_array($replace) ? array_values($replace) : array($replace);
         $replacements = array_pad($replacements, count($searches), '');
         foreach ($searches as $key => $search)
         {
            $parts = mb_split(preg_quote($search), $subject);
            $count += count($parts) - 1;
            $subject = implode($replacements[$key], $parts);
         }
      }
      else
      {
         foreach ($subject as $key => $value)
         {
            $subject[$key] = mb_str_replace($search, $replace, $value, $count);
         }
      }
      return $subject;
   }
}

As said there, str_replace is safe to use in utf-8 contexts, as long as all parameters are utf-8 valid, because it won't be any ambiguous match between both multibyte encoded strings. If you check the validity of your input, then you have no need to look for a different function.