After not having upgraded a Debian server for a while (1.5 year) I did an upgrade today. Everything still works, so that's positive, but for some reason imap is being slow, even though it's all local. It does respond instantly, but when using a webmail client it generally takes about 6 seconds to read a folder. When opening a mail it's there instantly.

Turned on debugging, and basically logging in is done instantly, but then when the webmail client does a SELECT INBOX, or a select of another folder, nothing happens for up to 5 or 6 seconds, after which the imap server finally replies and the rest of the session is handled off pretty much instantly again, so including message headers in the selected folder and that kind of stuff.

Can't think of any reason why it is so difficult and slow to switch to a specific folder, while everything besides that is quick and instantly.

Any ideas on what the reason can be? It's a postfix+courier setup that uses maildir and virtual users.

Let's see, you've given us your MTA, IMAP server, mail storage type, and user configuration... but NOT the slow web mail software!

That's because it's the same for all I've tried

I use Squirrelmail, Roundcube and Horde, all three have the exact same problem, altough Horde generally seems to be slightly faster.

All three have the delay in the exact same place.

Well, ok, but we don't know what you've tried, what works and what doesn't.

How about another non-web MUA to eliminate apache and the webmail from the picture? What's that performance like?