Not slow as you describe, but my PC in general is slow after upgrading this week (using auto-update) to the latest release (I was happily running the previous one).
Opening files and folders, even the Start menu, is suddenly really sluggish, though Avast isn't showing any particular CPU over-use or anything like this.
This has happened to me before I think. Another update destroyed Windows altogether last year, though when I rebuilt my system and installed that version again I was fine. I am losing faith in the incremental update method and am at this moment downloading (painfully via modem) the new setup file so I can remove and re-install Avast.
By the way, it isn't good to suddenly run a PC for a (short, believe me!) period with no AV installed - apart from the obvious reason - as it makes you (well, me anyway) sick to realise just how much the performance drops with AV running, even at the best of times. It's like someone just added a whole bunch of mb/sec to to my hard drive! Ouch! I did have to do this to prove to myself that it really was Avast that was suddenly slow. It is/was!!
I'm annoyed at all this and especially at myself. Let me remind myself here in public and also remind others; backup your system often and especially before updating or changing fundamental components like Avast. These things can really screw up on you!
Hopefully in a half hour I'll be running smootly again, if not I'll be back asking what the heck they did in the new release!!