I also was having problems with Avast20.4, but only my Win7x86 build. This is on a Pentium D820 desktop. On my Win10 computers, 20.4 works OK
It was taking 3 minutes to shutdown the PC. Event Viewer pegged the cause as Avast not properly closing. Removing components I don't use did no good. Repair did no good. When I uninstalled Avast, shutdown took 20 seconds. AVG20.4 had exactly the same problem.
I restored a backup that had Avast 20.1. The Win7x86 build now shuts down in 20 seconds. I disabled automatic Program updates.
I restored a backup that had Avast20.3. Same bug as with 20.4. I didn't have a backup with Avast20.2
It is not easy to roll-back Avast versions. If you use the online setup file for a prior version, it immediately updates to the lastest program version before you have a chance to disable it. What's needed is an offline installer with the prior version, but I couldn't find a source.