Cheshire Cat
The most curious thing..
You did.This sounds like an issue others, including me, had a few weeks ago - there were a few threads about it at the time.
For me, the issue persisited regularly in Safari but if I switched to Chrome on the same machine, the site was lightning quick, which indicated it was an issue at my end, not the NSC server end.
In most cases clearing your browser cache of NSC data (or clear your entire browser cache) should resolve the issue - as it seems something is lurking in there which is causing the problem.
Some people had a different issue last week, but this was isolated to those who use TalkTalk as their ISP. I did some delving into the TalkTalk support forums and found another TalkTalk customer who had just reported a very similar issue with another site. In common was that both his site and NSC use the same security platform - Cloudflare - to provide protection. In this case, unplugging your router for 30-60 minutes and then re-starting it fixed the problem.
I was the TalkTalk issue.
Unplugging the router for a hour worked for me. Everything magically returned to normal.