I've noticed with most of the AV systems I've tried over the years that they add a slight bit of input lag. I'm not talking about system stuttering or FPS drop in games. The mouse movement seems sluggish and inputs seem delayed a bit. For the longest time I thought it was in my head but if you google Avast input lag, webroot input lag, etc., you'll find numerous instances of users claiming that the AV system has caused a variety of system performance issues including strange mouse cursor movement. For Avast - which had a rather sever input lag bug associated with their game mode - one of the websites I read claimed that the issues was cause by the hardware assisted virtualisation setting. So, the performance impact of an antivirus system should be measured beyond things like CPU usage, fps drops, and network slowdown, but I think testers should also measure the general system latency impact that an anti-malware system has. Unfortunately this isn't an easy thing to do as you need a setup like the popular YouTuber nobattlenonsense has with a high speed camera measuring the time between the movement of the mouse or the input on the keyboard and when it is detected on your PC. As it stands right now, if you game seriously you're far better off running without any security software because on the bugs they tend to introduce and the system latency they can sometimes cause.