You will need to provide evidence of the issue. I would load this add-on and post a screenshot of the issue. see screenshot

What site(s) are your going to that load slow for you? Are you on WiFi or plugged in? Who are you using for DNS? Have you cleared your browser cache? https://support.mozilla.org/en-US/kb/how-clear-firefox-cache Do yo have the same issue with Firefox in Safe Mode? https://support.mozilla.org/en-US/kb/diagnose-firefox-issues-using-troubleshoot-mode#w_how-to-start-firefox-in-4troubleshoot-modesf5safe-modesf

Example... Lexus from Japan took over 5 seconds to load for me. see screenshot

try to clean cache maybe too much got saved up on your browsers files or redownload your browser

This problem is thankfully solved. It was the MalwareBytes addon which slowed down my system. Thanks for all your help!

Yes, I've recently seen several reports that Malwarebytes Browser Guard is slowing down Firefox, so maybe contact them.