SOLVED: Very slow loading for specific websites. speedtest.net in particular
I figured out my issue while writing up the problem report. I'll share it now, so that others may benefit from what I learned. In short, my VPN provider (NordVPN) includes an ad-blocker, and it appears to add 30 seconds of latency for completing an HTTP request to a blocked site. Interestingly, the blocker triggered this long pause in Firefox but not Edge or Chrome.
This is 100% repro on my Windows PC. I have Troubleshoot Mode, and after resetting the browser. It never repros with Edge and Chrome on the same PC.
I looked in Developer Tools and captured a recording of the poor performance. There are several HTTPS requests to CDN addresses that each take a very long time to complete. Each is suspiciously 30,000ms plus or minus 11ms. The profile is at https://share.firefox.dev/3BYtGqm.
When it finally loads, I can run Speedtest to measure performance. It shows 870Mbps down, 598Mbps up, 6ms idle latency, 21ms download latency, and 4ms upload. This is gigabit, Ziply Fiber, and a wired LAN.
Other sites load quickly.
How and what VPN provider does it up to them FireFox has no control how they filter Browser and you should contact them about this issue since they are the VPN support to fix or change what it's doing to the FireFox browser that connects through the VPN.