I installed avast free antivirus on an old computer from 2009-2010, my internet connection is much reduced, 1000 MB/s in download and 500 MB/s in upload, with avast I have about 450 MB/s in download and less than 500 MB/S in upload. It's like that in all kind of internet connection test, speedtest app for windows, my ISP test in browser, speedtest in browser and nperf. During the test with the speedtest application of windows the needle lag, it is not fluid, the mouse cursor lag too! It's horrible!
When I uninstall avast antivirus or if I put it in passive mode I have the right internet speed, no more problem. I tried avast one, it's the same! For the 2 I tried to deactivate all protection agents, it does not change anything always a loss of internet speed with lag.
On my recent 2019 computer I never had any issues with avast antivirus.
I use avast with secure browser in bank mode, and I was going to look for another password manager because that of avast and EOL. But I think I'll give up avast for windows defender antivirus, another password manager, and keep avast secure browser without bank mode, this on my 2 computers. This is not what I prefer I would like you to solve this problem.