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.