phpbb\extension\manager\all_available() and others like it use the RecursiveDirectoryIterator to search for \ext\VENDOR\ext.php (and composer.json) files, but it does so with no limit on the depth search. This can take a long time to load for some extensions that include many files.

Possible change the code to make use of the finder instead, which might be able to restrict the depth to just /ext/VENDOR/.