I never close anything, I frequently have something running on my TV from my PC while I'm playing a game of some description. Never really get any problems with anything slowing down and if I do I just go turn off whatever it is that is causing it. I definitely don't go the effort of finding and closing background processes etc, did it once or twice and noticed 0 performance increase. Especially considering most games are GPU intensive rather than CPU, adding a few background processes for the CPU to handle while gaming makes no difference.