get_page_templates consumes too much IO for each page request

Hi,

Please add a transient cache for get_page_templates if it sounds ok to you.

I presently have a auto-scaling setup with a network drive (AWS EFS) and this slows down each page considerably since opcache does not cache file reads. Using fatrace I observed that php files in my theme were constantly being Opened->Read->Closed by PHP-FPM. I traced it back to this plugin.

My original time reported by query-monitor was 2.45 seconds.
After I added a transient cache it improved to 1.14 seconds.

It’s uncommon for page templates to be added/removed frequently but if you add a transient here it would need to be documented to prevent confusion.

Example:

$templates = get_transient('ao_crit_page_tpls');
if(!$templates) {
      $templates = wp_get_theme()->get_page_templates();
      set_transient('ao_crit_page_tpls', $templates, 3600);
}