I never had issues like this with McAfee VirusScan 8.6, so I wonder if joelclark was actually referring to 8.6.1. I recently started testing McAfee VirusScan 8.6.1 and had a similar experience to what joelclark described on a couple of machines: an iMac G5, and a 533 MHz Power Macintosh G4 with 512 MB RAM, both running Tiger.
On the iMac G5, the system became painfully slow immediately after upgrading from 8.6 to 8.6.1. After rebooting, the system seemed to be running normally, VirusScan 8.6.1 updated its definitions properly, etc., so I decided to leave 8.6.1 installed. This was a couple days ago and I haven't heard any complaints from the user, so I believe 8.6.1 is still running fine on that iMac G5.
On the PowerMac G4, the system was also unbearably slow after installing 8.6.1 (in this case, 8.6 had not been installed on the system previously). After restarting the system, the processor was still pegged at 100%. I opened the Terminal to investigate, ran the "top" command, and found that several instances of VShieldScanner (whose name is truncated to "VShieldSca" in top) were running and were responsible for the extreme processor utilization. I rebooted a few times after that, with the same results. I disabled On Access Scanning and restarted, but surprisingly that didn't fix the problem. I tried uninstalling 8.6.1, rebooting, and then reinstalling it, and that didn't resolve the problem either. Next, I let the machine sit idle (other than the top command and LoadInDock) for 45+ minutes thinking that perhaps VirusScan was actually doing something and would eventually finish its tasks, but it never did; in fact, it got worse! As many as 24 instances of the VShieldScanner process were running by the time I decided to give up and uninstall again. After I uninstalled VirusScan 8.6.1 that time, I rebooted and then installed 8.6 for the first time on that system. Right after installing 8.6, I was dismayed to see that the VShieldScanner processes were back and were hitting 100%, but I decided to restart, and this time the system went back down to normal processor utilization just a couple minutes after logging in. Right now it's idling at between 6% and 21% processor utilization, which is normal behavior.
I have an Intel Mac (MacBook Pro 2.0 GHz Core Duo with 2 GB RAM) running Tiger on which I could test 8.6.1, but I use that system a lot so I'm hesitant to install buggy anti-virus software on it. McAfee VirusScan 8.6 is currently installed on that system and it's running fine.
So to summarize, for the iMac G5 all it took to resolve the problem after installing 8.6.1 was to just *restart the system and wait patiently for it to reboot*. However, the only thing that resolved the problem on the PowerMac G4/533 was to *uninstall 8.6.1 and install 8.6 instead*. I haven't tested 8.6.1 on an Intel Mac yet.
Hope that helps!