Nico Schlömer put forth on 9/14/2010 1:51 PM:
The IMAP server in the present setup is installed in some physical distance to the RoundCube server I believe, it's running some Microsoft thing and generally I would say is not well administered. This could potentially be the culprit (but wouldn't explain why it's still slow with caching enabled). Is there any way I could test or benchmark this?
With no details of the server available, the only other thing you can do on the RC box to possibly increase performance is install imapproxy, which will eliminate the constant log on/off cycles of this stateless web app.
You do realize that every mouse click on an item or folder in RC (or almost every click) sends an IMAP login to the server, and a logout after the command has completed, don't you?
logon -> command -> logoff for just about every mouse click in RC
This constant authentication and IMAP process creation/tear down put one helluva load on an IMAP server, assuming it's not a hobby server, and likely especially, any MS server running a Win32 IMAP server. If the IMAP server is the Exchange component, doubly so.
Given the fact your RC server is remote, I'm kinda shocked you don't have imapproxy installed already.