PHP 7.4 introduces opcache.preload, which allows code to be preloaded. This may be highly compelling given Phabricator's architecture (where all loadable code is known prior to request startup and loading code is side-effect free), as discussed in more detail in T2312. At the time T2312 was filed, we were running a modified version of php-fpm that I'd patched to have similar behavior.
There's some minor subtlety here (do we want to load src/extensions/?) but this is probably and easy change with substantial performance benefits.