Well, after "Repairing" & re-enabling that "HTTPS Scanning" option again, at least a youtube video page loads in about 8 to 9 seconds now instead of hanging for minutes, but still the Web scanning engine has a HUGE impact on pages that load lots of small items, like youtube for example (load in 2 to 3 secs with Web Shield disabled) , and it's most evidently when there are lots of items on HTTPS sites.
Web Shield still makes average web pages loading time 3x as long as with it disabled.
@Para-Noid "Repairing" Avast at least seem to have had some positive impact. Thanks a lot !