[imp] Imp Performance Problem

[Registration]

Dear Imp users and developers,

my problem is as follows:
- Login to Horde takes 3 seconds.
- Loading mails takes 7 seconds for a larger IMAP folder, and 1-2 seconds for one with only a few messages in it.
- Top shows 97% CPU from php5-fpm, 3% from imap.
- When messages are cached, everything gets super fast.
- The loading times are probably proportional to the amount of message headers Imp loads for viewing.

My questions are:
- Is this normal speed?
- If not, how would I systematically search for the problem?
- I tried xhprof, but I can't get the part where Imp refreshes messages captured.

My config:
- Debian Wheezy, Apache, php5-fpm, Dovecot, imapproxy
- Auth using Imp using MySQL/PDO connection via sockets
- Dovecot using Maildir, PAM disabled, mysql auth only
- Horde caching enabled
- everything on one host
- standard rented server
- everything up-to-date