[cannam]

I think you're right about this one. Sorry about that. It seems to have started interacting badly with Ghostery -- which I had been using for years, but which did have an update recently as well. I tried a different ad-blocker and it's been a lot quicker since.

I think I jumped to conclusions because the change roughly coincided with a visible update to FastMail (the one that added the little progress spinners to the folder tree).

[cannam]

I'll add, for the record, that simply trusting the site in Ghostery appears to be enough to speed it up again.

(My earlier post lost a couple of HN points after I posted that reply, even though it was many hours after the original discussion. I wonder whether people had upvoted it because of potentially useful material about Firefox and FastMail and then unvoted when I admitted that particular bit was bogus. It would be rather encouraging if that were the case)

Later edit: I spoke too soon. After editing an email extensively, the whole UI slows down again.