I have never had a problem with ZA (that I couldn't fix). It does not slow me down more than 5% tops.
Well i had the 2006 internet security package and as soon as i installed it download went from 4000Kbps to 2000Kbps. I even tried lowering the security settings but couldnt get it to work.
PC-chillin keeps me going at 4000Kbps without a problem. I also ran that active secuirity monitor thing and scored a 95 so i guess it gets the job done.