Avast! anti-virus, which is used by millions of people around the world, recently upgraded to version 4.8. Unfortunately, it seems that this upgrade has caused many WoW players to have a drastically lower play experience.
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.