The same problem here as the O/P with Avast A/V (version 17.5.2302 (build 17.5.3559.0) and Opera browsers (Opera 46.0.2597.39 latest version on one pc, and 42.0.2393.517 on another pc).
The only difference from the O/P is, I’m using the following on my desktop pc:-
(1) AMD A8-5500 APU processor 64bit
(2) Windows 10 Home edition 64bit
I’ve tried all suggestions in this topic, although disabling the shields made no difference at all. In the end I uninstalled Avast A/V using “avastclear.exe” in safe mode. I then reinstalled Avast A/V, but no change, Opera is still painfully slow to open and use. For example it takes Opera at least four times longer to open than usual. Firefox, Edge, IE and Nitro browsers appear not to be affected.
This is not meant to be a criticism against Avast, more of a heads-up for the developers, as I’m sure there must be others like myself with this problem who are running Windows 10?