AVG does cause noticable slowdown. So do Kaspersky and NOD32. I keep AVG free around for one reason: Link scanning. I use MBAM for actual weekly virus scans (Which it is a lot better at than AVG. ie: AVG couldn't find Vundo, MBAM did.)
But the best defense against viruses and trojans in the first place is "Don't do anything stupid" Which does not translate into
"all pr0n has viruz lolz" I've only ever gotten a virus from a porn site once, and it was from doing something very dumb. (Actually downloading something)
Edit: Just for the record, I have never used a firewall, and have never needed one. They generally seem to cause more problems than they're worth.