I read your post and my computers present the same systems described; lag on some pages and when watching YouTube videos.
I did the steps you described to me disabling the QUIC / HTTP3 scan and it works. Thanks a lot
In addition, as another alternative, it would be to install an older version of avast free, for example, 20.9 or 20.8 that do not present this problem. For those interested, the standalone versions can be downloaded here.
https://www.filepuma.com/download/avast_free_antivirus-2/versions/It must be installed without an internet connection so that the program does not automatically update to the latest version.
Once installed you have to go to settings >> (In the "general" tab go to the option that says "update") >> click on the option "Your application" >> click on "More Options" and select "Ask when updates are available ”.
Then they can connect to the internet, just ignore the messages that the application needs to be updated while Avast releases a new update that corrects the problem. In this case there would be no need to disable the QUIC / HTTP3 scan since the previous version does not present this problem.
I apologize again for my bad English.