Ok, the depth problem might be less simple than I thought first. Thanks for your help all of you, I think I'll stop for today. The next step (apart of this depth issue) will be to find out what causes the remaining occasional slowdowns. What would be useful would be to find some scenes that show this behaviour, and if possible the exact moment in the scene where it starts to get slow. The best would be scene that run at normal speed and an effect in that scene that is triggered by the player and causes such a slowdown, so that I'll be able to see what in the effect possibly could be the culprit.