Does it make a performance difference?
I tried benchmarks:
Counter-Strike: Source - Anti Virus ON: 271 FPS - Anti Virus OFF: 270 FPS
MSI Kombustor - Anti Virus ON: 37 FPS - Anti Virus OFF: 37 FPS
It didn't affect performance in the benchmarks, but games probably don't work the same way as benchmarks. Is it a good idea to disable Microsoft Security Essentials before starting my games (singleplayer)? Would it give me any performance increase?
look, Anti-Viruses are designed so that they can work in the background and uses minimum resources so that even if you have a crappy laptop made in 1997, you can still be able to use it and yar-har-fiddle-dee-dee your favorate albums and movies online. they run through only a few mb per second when nothing is newly written, not taking your whole hard drive and shove it down your CPU's throat.
Unless it's a Norton 360, I don't see the reason on why you should disable it to get better performance. Other or today's antivirus programs doesn't use much of the resources and still provide good results on scanning. And to answer your latest question, no, it won't have a huge impact on performance. ( i.e , if your doing a full system scan, then you'd notice a decrease in your performance ).