best of them is sandboxie and it slows the hell out of browsers, tried it with firefox and chrome, same thing with both.
It would be more accurate to say that it has that effect on YOUR browsing. Not on mine... not with Chrome or IE9/IE8, XP or W7. Not in the least.
The only slowdown I find is of launch times which are roughly doubled, all else is unaffected. Every security measure reduces performance in some respect, whether it's worthwhile is a cost/benefit thing.
One of the reasons I use Sandboxie is, there is no drag like with AV's on my older XP-SP3 machine. I tried a few free AV's and they are fine for a couple of days and then I feel a slight drag.
I have the similar "one of the reasons" of Mild_Manered; this "one of the reasons" is enough to not use any AV now. SandboxIE and Toolwiz Time Freeze are an excellent duo in my security setup.