Installed Avast home edition on a friends computer. He had not ever done anything with it. He purchased it in 1998, dell 500mhz slot Pentium3, 64mb pc100, basically your run of the mill old system. He had the original trial version of Macafee that he never updated. Never defraged(took hours), or cleaned the registry(almost 1000 broken registry links and hundreds of broken shortcuts and invalid uninstallers), and he was using AOL. Thats were I came in he wanted high speed internet. I removed AOL, macafee, and installed system mechanic, and Avast! as well as sygate, spybot(found 4 problems), ad-aware(found 134 problems), and spywareblaster.
The point is that after all the work the computer was faster and Avast did not slow this old system down. I have heard some people say that Avast slows, slow computers to a crawl. This computer was an EXTREME example of misuse and negligence, and AVAST! did not effect it at all.