I hope to not muddy up this thread but I thought it might be helpful to let you all know that I ran the Windows upgrade from 8.1 to windows 10 64bit after encountering this problem on my HP 17-f023cl laptop.
Here is my hardware detail;
HD: Hitache Travelstar 5K1000|HGST Link to HD detail:
https://www.hgst.com/products/hard-drives/travelstar-5k1000RAM: 8 GIG
CPU AMD A10-5745M APU with Radeon HD Graphics 2.10 Ghz
I have found that if I immediately go into my services as Windows is starting and disable Avast, wait for the HD activity to drop to less than 5% (takes about 1 minute after disabling Avast service) and then I restart the Avast service, the performance issue goes away.
This work around worked for me in Windows 8.1 (patched current) and it works for me in Windows 10 (patched current).
I've been upgraded to Windows 10 for a little over a week now.
I also run this same version of Avast on my Windows 7 laptop that is about 5 years old and I do not get any of these performance related issues.
I would be happy to share any additional information with the group and/or support to resolve this issue as I would really hate to switch scanners after loving Avast for many, many years now.