Kaspersky can cause performance issues if you have "Inject script into web traffic to interact with web pages" enabled and don't have the companion browser extension installed.

Thanks, but what is Kaspersky?

Oh, it's an extension ... I don't have Kaspersky, so that's not the issue.

Does it still happen in Troubleshoot Mode?

Yes, it's happening in Troubleshoot Mode. But I've clarified the problem. When I open Firefox from scratch, or open a new window, the first site takes 15+ seconds to load. But thereafter, new tabs in the same window open quickly. And to clarify: this isn't happening with either Chrome or Edge. It's solely a Firefox issue.

I guess no one can help me figure this out.  :-( Back to Chrome?

Does it still happen in a new profile without signing into Sync? An easy way to test a new profile is to install Developer Edition and see if it happens there or refresh your existing profile.

Thanks! I figured it out. An extension called Privacy Pass was the perpetrator!