I think the secret with AVG 8 is to use the "manual" install, and NOT to install the browser toolbar. That seems to be what slows it down.
I've been running AVG for years, automatic updates and automatic scans every night.
Last night, I updated to AVG 8. This morning, AVG 8 found 87 viruses, trojans, spyware, etc on my machine. None were "installed" - most were in e-mail attachments that I hadn't opened but had kept as "evidence" if I ever needed them. But AVG 7.5 hadn't seen them. So updating seems like a good idea.
There were also dozens (literally) of spyware entries in the ActiveX setup on Internet Explorer on this machine. Since I don't use IE, that didn't make much difference. But I thought it scary all the same. Firefox was, apparently, clean.