I can't explain this one. EvC Forum is a secure site - http requests are automatically translated into https requests.
On a positive note, after not being able to find anything wrong yesterday I resolved to return to the issue today and found performance back to normal this morning. I'll return to investigating the problem if it returns.
A server control panel setting looked wrong so I switched it back to what I thought it should be. It should never have changed in the first place, so I figured I must be misremembering, but then it worked. Hopefully that was really it and the problem is really gone.
The problem returned. I checked into the possibility of a Denial of Service attack and found that something called ClaudeBot was banging on EvC very hard. I've blocked it. Apparently I made the earlier change at the same time that ClaudeBot was taking a break.
The problem didn't recur while I happened to be online, but I'll keep my eye out for performance issues. If it happens while I'm online I can track the cause.