@danpost Thank you so much! Your 'guess' is proven by this image btw: http://answers.unity3d.com/storage/temp/984-wrapping+world.png

Well, your computer will still need to run those Actors' act methods, it just does it simultaniousl. So yes, it could help, but only if those act methods take a noticable time to run. Also, as davmac already pointed out, Greenfoot isn't thread safe: to do it completely safe you'll have to look at his scenario: http://www.greenfoot.org/scenarios/1963 So if you need the 'performance boost', I suggest you also check out his scenario and use it!

I find it really weird, especially since I don't seem to do anything weird except for running it in another thread. Sorry for the inconvenience. Maybe it's your computer, although I can't say that for sure. It's just that you're the first one to experience this.