I am experiencing extremely low FPS (25-30 under normal conditions and about 12-15 in team battles)

My machine is very old, but I am curious, because I am getting same FPS on THE lowest settings possible AND on the highest settings possible..

That's not all the bad. It's either your machine, or you're running too much stuffs in the BG.

My first guess would be background processes. Try turning off some unnecessary applications. Type msconfig into Run... to access a useful tool for disabling automatic processes that you don't really need.