The most common symptom is severe keystroke lag while playing the World of Warcraft. Mouse actions also seem to be affected by this. Delays as low as a second and as high as five seconds between keystroke and the game receiving that action have been reported. Supposedly, running WoW in windowed mode fixes this, but your mileage may vary.
There is no definite, official statement that I've seen declaring this to be the cause, but there seems to be enough anecdotal evidence pointing to either Avast! 4.8, or a conflict between Avast! 4.8 and some other hardware/software. I've personally been witness to someone troubleshooting keyboard lag in the World of Warcraft, and it turns out the problem was Avast! 4.8, and the issue cropped up immediately after the update of the software. Removing Avast! instantly removed the lag.
A full uninstall seems to be what alleviates the issue, but I don't recommend doing this unless you have other virus protection in place. I use AntiVir, but AVG has been another popular recommendation for an alternative.