It takes up to 55 seconds for any request to come back. Login, Viewing a Message etc, Switching a Page .... -- Would use the opcode cache, but I have a feeling there is another problem somewhere else. Caching is on.
Let me know what config files you would like to see - I'm at the end of my rope.
I've resolved this issue and have written an article summarizing all the methods I've used to speed up roundcube. Some haven't really been discussed in these forums - particularily - Javascript Optimization and DNS Resolution Bottlenecks
http://seal-7.blogspot.com/2007/03/improving-performance-with-roundcube.html