PHP 7.4 ships with opcache preloading - https://wiki.php.net/rfc/preload - which allows a framework to specify a preload file which will:
...load a certain set of PHP files into memory - and make their contents “permanently available” to all subsequent requests that will be served by that server.
All the functions and classes defined in these files will be available to requests out of the box, exactly like internal entities (e.g. strlen() or Exception). In this way, we may preload entire or partial frameworks, and even the entire application class library. It will also allow for introducing “built-in” functions that will be written in PHP (similar to HHVM's sytemlib).
Notably, this comes with some drawbacks that may not make preloading advisable/possible in certain environments (e.g. shared servers) but is well suited for more "modern" runtime environments such as containers, which generally map to 1 "server" (container) per codebase.
The traded-in flexibility would include the inability to update these files once the server has been started (updating these files on the filesystem will not do anything; A server restart will be required to apply the changes); And also, this approach will not be compatible with servers that host multiple applications, or multiple versions of applications - that would have different implementations for certain classes with the same name - if such classes are preloaded from the codebase of one app, it will conflict with loading the different class implementation from the other app(s).
Akin to Symfony 4.4's compatibility: https://symfony.com/blog/new-in-symfony-4-4-preloading-symfony-applicati...
The proposal would therefore be to ship a preload file which may be optionally included in the site's php ini configuration set; this would be a default-OFF (since it requires manual intervention in the hosting environment) and opt-in ON situation.
As Drupal has much of its core imported from Symfony and already boasts an extensions API, I would imagine extending Symfony's model would be a starting point.
