My experience is that Symfony, in its fat, batteries-included form, does take quite long time to initialize on cold caches. The first request can take hundreds of milliseconds but usually the very next request is in the normal 10ms range. This is especially noticeable on (cheap) shared hosting which has always been a common place to run PHP.
I've never found it to be a problem on a VPS but if you're developing on something slow like a Raspberry Pi I can see this happening regularly. If you're used to deploying stuff in containers and constantly throw out the cache the initialisation problem can happen very easily as well.
IIRC back in symfony 1 part of it was becasue symfony used the first request to write optimized versions of the code into a folder that I cannot remember the name of.
I think there was a way to do this up front in Symfony 1 and there might be some way to do this as part of a build and deploy step. Of course on applications with less traffic and less strict requirements just hooking something up to run a request immediately after deploy might work just as well, but if it runs across n small pods it might be a much better idea to do it on a beefy build machine instead of on n resource constrained pods causing slow loading for n customers.
I've never found it to be a problem on a VPS but if you're developing on something slow like a Raspberry Pi I can see this happening regularly. If you're used to deploying stuff in containers and constantly throw out the cache the initialisation problem can happen very easily as well.