I've got a folder (netscape.public.mozilla.mail-news) with 13,000 messages. switching back to my small inbox (45 messages), is very slow. something to add to our perf list....

Try downloading php.dev from news.php.net, over 70,000 headers. You don't even need to open the messages to see Mozilla freeze up for five minutes just switching to the big folder. sspitzer -- do you see the performance hit going into the folder, or coming out of it?

I do not know if this is related or a separate problem - if I have a thread that contains several hundred messages (all on "child level" 1, e.g. no nesting), Mozilla freezes for a few second when asked to expand it.

these are three separate problems.

Karsten, Tony, for the trivial case of comment 0 I think this can be closed. Would you agree?

given that a) the problem will have been a core issue that wasn't identified and b) we don't have anyone still active here with a testcase ... opting to close - existing core bugs likely cover all the open issues => incomplete

The most likely cause of this is the time it takes to free up all the memory associated with a 13K message folder. There's not much we can do about this, other than using memory arenas in Mork (e.g., PL_Arena), which makes the freeing of lots of chunks of memory a constant time operation.