I'm wondering if keep-alive settings affect how well roundcube performs. I was concerned because I have my web server behind a firewall which keeps the keepalives low.
I just installed up-imapproxy, which caches IMAP connections and it has helped a lot. It also helps with Squirrel Mail. :-)
Maybe both Circle Box and Squirrel Mail could benefit from Cache_Lite as well - I've found it to be an excellent resource for improving web application speed.