Avast Internet Security, paid version
Since July 14, 2017 ? opening webpages and videos is extremely slow or impossible. Also my automatic BackupOnline is not working.
Pc with Vista, had never any problem before. Same in all browsers I tried (Firefox, Chrome, Opera)
Problem is solved when all shield are off!
Only Avast firewall is compatible.
Tried reinstall, first with free version, but no solution. What happened??
As has been explained many times already, disable ssl/tls scanning in the webschield.
Hi 
Put your Shields back On!
Open Avast
Click On… Settings
Click On… Components
Where It Says - Web Shield : Click On… Customise
Un-Tick… Enable HTTPS Scanning
Then your internet will work OK!
Lets just hope there is an update to put this right soon.
It should not be my task to do this! I bought something that worked perfectly for more than 10 years, and suddenly there are problems. It took me 4 days of digging in my computer before I discovered the least logical source of the problem: Avast.
Why is this program checking https anyway?
But indeed this is the solution: disable https scanning.
Once again this should not be my job, but should be fixed by Avast without any delay.
It will be fixed by Avast. Till that happens, this gives you a workaround.
Your computer can get infected via HTTPS just as it can via HTTP. Keep this in mind while the HTTPS scanning is disabled.
This bug seems to only affect Avast on Windows XP and Vista. Perhaps that is why no fix has been forthcoming thus far.
This has now been fixed, at least for me. 
A “micro” update has been released to address this - I think it was installed either when the PC was booted or when I updated the virus definitions. Try turning your HTTPS scanning back on to see.