I just did a clean install of Windows 8.1 on an Acer laptop followed by an install of Avast free.
I followed that with 123 updates. No problems with the updates or with Avast.
I can't recall ever having to totally update the OS before installing Avast.
There is a time after Avast is first installed that things are a bit sluggish but that's only for a few minutes
and depends on the OS and the additional modules installed or omitted.
If you're still having that problem after all the reboots, maybe a repair or clean install of Avast may be in order.