when running my game the performance starts out fine then it slowly gets worse and about 10 minutes later the performance have dropped quite notably (goes from about 512 fps to about 400 and gameplay is alot slower).
i have tried to figure out what is causing it but havent managed to.
i would like to hear if anyone have had a similar experience or maybe have an idea what might cause this performance drop over time.
it happens even if im just standing still and doing nothing at all.

First of all, you have to be aware, that your code is framerate independent! After that, you need to set the "fps_max" to "60", which is probably the best for the most type of games. If changing of the fps affects much on the movement (rotations etc), this means that you've coded your game wrong.

Read manual about this, and probably there was something about it in the workshops, take a look as well:
http://tutorial.3dgamestudio.net/
http://www.conitec.net/beta/atime.htm

well the game is framerate independet as changing the framerate from 512 to 60 doesnt affect gameplay speed at all. however somehow the performance seems to stay the same after i added an fps limit. seems a bit weird but if it works then im happy.

check the debug panel (F11)....maybe you create entities in an endless line.

The panel also shows what eats the most milliseconds.

Denn15@ there is nothing weird actually, the fps will go down as soon as your project gets complex (but you have to be aware to do not spent memory on nothing). If fps is higher than 60, than you are good to go

well amount of entities are the same all the time. somehow setting the fps limit at 60 fixed this so it works fine now

Please keep the dimension in mind we are talking about here. When you get 400 - 512 frames per seconds that means that render a single frame took between 0.4 and 0.51 ms.! That is very very constant.