Optimize performance of custom pre-built option list to webform
You can always cache the results as seen in the official documentation

  if ($cache = cache_get('my_module_cache') &amp;&amp; REQUEST_TIME &lt; $cache-&gt;expire) {
    $data = $cache-&gt;data;
  }
  else {
    $data = 'xxxxxxx'; //do all your time taking complex calculations here
    cache_set('my_module_cache', $data, 'cache',  REQUEST_TIME  + (3600 * 24 * 30 * 6)); //stores in cache table and expires after 6 months
  }


More about cache_set and cache_get