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#11
A couple of things. What's your antivirus and does it have SSL scanning? If so disable SSL scanning temporarily or else import the certificate if missing.
I had the problem with Kaspersky with various browsers.
Here's FF SSL settings from my machine for comparision.
https://www.howsmyssl.com/
Also try changing DNS to see if it makes any difference.
Very slow internet on Windows 10 Solved - Page 2 - Windows 10 Forums