Hmm . . . . I had the same problem. I let the page finish loading and had less hesitation. Then I noticed that whenever the US map was displayed, the problem returned.

I used "Inspect Element" and removed the map and the problem was gone. So the map is the problem.

No problems here with the Johns Hopkins map (lots of content blocked by Tracking Protection). The map is a SVG image with 3058 nodes.

Start Firefox in Safe Mode to check if one of the extensions ("3-bar" menu button or Tools -> Add-ons -> Extensions) or if hardware acceleration is causing the problem.

• switch to the DEFAULT theme: "3-bar" menu button or Tools -> Add-ons -> Themes
• do NOT click the "Refresh Firefox" button on the Safe Mode start window

• https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-using-safe-mode
• https://support.mozilla.org/en-US/kb/troubleshoot-extensions-themes-to-fix-problems