Did some tests and would like to share the results:

The problem DOES occur on another machine with a plain setup (no Docker, fresh PHP 7.4 install, Xdebug 3.0.4) with the linked simple Symfony app.

The problem does NOT occur when downgrading to Xdebug 2.9.8 using my real-world application (and otherwise unchanged environment). Symfony container build time goes down to ~12s which is expected. As the container grew in the meantime, build time went up to 158s with Xdebug 3.0.4 (with 12507 calls to ReflectionMethod::getPrototype using up 129s = 81% of the wall time according to Blackfire)

Thanks again for your help!

Just acknowledging that I've seen the issue, but I didn't have time yet to try out your docker container.

I suspect that because Xdebug 3 removed a bunch of settings, it is always doing more work creating stack traces. I suspect that the Symfony methods here might include very big objects etc in the function call that happens many times. You can try setting https://xdebug.org/docs/all_settings#var_display_max_depth to a lower value (1, instead of the default 3) to see if that makes things better. If you only care about debugging with Xdebug, then you can set xdebug.mode=debug, and then all the work that Xdebug does in 'develop' mode should not be done.

@derick: Thank you for your reply, that's the right direction. I found that we even configured var_display_max_depth = 10 (maybe I'll find out why) so that seems to be the reason why this problem hurt us more than others. When reducing the value to 3, build time is reduced by half, and when reducing to 1 it's 11s compared to 6s without Xdebug.

Disabling debug mode and setting var_display_max_depth = 3 will reduce wall time to 9s.

So we will definitely use some of your suggestions. If you can find the time I still appreciate optimization on Xdebug's side so that it's harder to shoot oneself in the foot. In any case thanks again :-)

I have already done most of these optimisations with Xdebug 3.0, so I will be closing this issue. There will always be ongoing work on making Xdebug more performant.