Just registered to say that i experience the same issue with avast making my system slower. I have fresh installation of windows 8.1 pro 64bit(all important windows update files installed) with latest version of avast and i can see avast slow effect even when i right-click on desktop. With avast turned off - i see context menu immediately, but with avast on - i usually have to see spinning blue circle first. I only run web and file shields, btw.

My system is:
Windows 8.1 pro 64 bit,
Core i5 3570k
GTX 760 with latest driver
8gb of ram.