Sure any antivirus is going to use the cpu for auto-updates.
In Win 7 and Firefox, using AMD System Monitor, I see that while playing YouTube videos sometimes my AMD R7 240 graphics card is pretty active, and other times it's not, during the same video (very strange!). I know that the default format on YouTube is their vp9, I think it's called, video format, instead of the more universal mp4, as it saves bandwidth at the same quality. In Win 7, offloading vp9 to the graphics card, if it's recent enough to include it, isn't supported in browsers. And vp9 takes more cpu (or graphics card power) than mp4 because of the greater compression at a given video quality.
I tried a browser extension which is supposed to force YouTube to use mp4, but it didn't make any difference. I really don't know which is playing when, if ever. :-)
I think a more recent video card, like a geforce 1030 that has vp9 hardware processing, which I actually tried but with no improvement, and Windows 10 (for browser compatibility with vp9) would result in offloading most of the vp9 processing to the card, but since I will be putting in the 8-core FX cpu, they won't be needed. That cpu should take it on quite well (since it has 8 cores, each of which is 25% faster than each of the Athlon II's 4), leaving plenty of cpu to give to Avast auto-updates and whatever else, ie rendering a video in Shotcut with Priority set on low in task manager.
Pretty complicated topic.