OK, once again - I'm not trying to tick anyone off, and I'll emphasize that I'm not an security expert; and I dare say it is a fact that I'm not alone in that!
I'd logically however, take the complaint that there's too much performance overhead very seriously. Take me as mainstream (rather than part of the statistically smaller security expert elite). My primary purpose in using the computer doesn't send my head into security issues every day. It sends me places and to activities in which great computer performance is a very positive thing, very noticable one way or the other, and economically significant.
Given that I haven't been plagued with noticable virus problems to any great extent (or my other software - not specifically labeled as anti-virus is taking care of them) - the net calculation is that a pound of cure, when needed, is worth a lot more than an ounce of prevention that's clogging up my machine all day.
Just one more thing - I really appreciate these freeware and open source things. I was plagued by unemployment a few years ago, and if it wasn't for open source and freeware, I would not have been able to use my computer at all. (SpyBot saved me.) I donated when things got better. So don't take my comments as anything other than an attempt to be helpful and constructive.
Of course - I always search the web and read reviews and make sure I'm downloading from the right place before getting anything.