Just had similar problems. Google searches worked fine. Many websites came right up while others would be painfully slow or not respond at all. Websites hosted nearby usually worked fine while those hosted elsewhere were sluggish or completely unresponsive. Tried changing DNS servers without any difference. Found that prefixing all websites with HTTPS:\\ in the browser regardless whether the website was a secured site or not alleviated the issue completely for all websites.
Turns out our Dell SonicWall firewall had done an auto update causing one setting to be changed causing the issue.