This comment is now just me explaining what we are currently trying to solve this.
Our current approach to solve this is by cutting away possible reasons. The first big divide we wanted to make is GameMode, UI, GamePlay classes (Character, Controller, etc.) and the Map.
We created empty levels, without the GameMode, just with a rotating platform.
→ NO FREEZE
Then we added the GameMode to that empty level, which gaves us back all classes of that GameMode and our UI.
→ NO FREEZE
Other devs of our project went ahead and removed all particles from the actual map.
→ NO FREEZE
Our next step is to find out if it’s really caused by some of our particles (or its actor?).