[imp] Slow loading of mailboxes with multiple folders and filters

[OVi C]

Hi. I'm using horde webmail edition version 5.0.4 and an user with a large
number of imap folders and filters (around 160) says that  it takes a
pretty long time  to display them in basic view.
The filters applied to the folders are ingo sieve scripts on a dovecot imap
server.
I'm using APC for caching and also memcached. Is there any way to speed up
the process of parsing the folders ?
We previously used horde 3 and he said, that back then this didn't happened.