I found the issue: Registering the same event handlers again and again in the tests, leading up to about 50 times the same code being executed (50 tests that did that). Fixed now.

How I got there:

1. In Github Actions, the tests also ran awfully slow, 40 Minutes for 970 tests. So the problem could not be locally.

2. I also disabled Xdebug, didn't solve the issue.

3. I started running the tests directory for directory. The summed up time for all tests were only 2 minutes 20 seconds.

4. I also noted times for some longer running tests to find after which tests the performance problem started.

5. This helped me identify The tests which caused the problem and then debug it.