Click F2 to check what sort of frames per second you are getting, if you are getting low FPS then this could be the problem.
Also some maps with fog and lots of props can make people, who use low end PC's, lag a lot. Check your CPU usage in the task manager and close anything that is using too many resources.
I did what you said and got rid of processes. It still doesn't change the fact that it's super slow during battles.