Show proof of the issue.

What is your load time for lexus.jp ?

Here is the add-on I'm using to show load time. https://addons.mozilla.org/en-US/firefox/addon/load-time/?utm_source=addons.mozilla.org&utm_medium=referral&utm_content=search

Who are you using for DNS?

What security software besides MS Defender are you running?

If you're running Windows Pro version, I would test in the Sandbox to confirm. https://www.howtogeek.com/399290/how-to-use-windows-10s-new-sandbox-to-safely-test-apps/

Thanks for responding, but there are no instructions on how to use the add-on whose link you sent... Running Malwarebytes for security and VPN; I don't have Windows Pro, so I can't use a sandbox and wouldn't know how to do so if I was.

Just install the add-on and then disable Malwaredoesbyte and vpn. Then see if the browser is slow.