I may be biased (:D) - but for myself RC is a lot, lot faster than Squirrelmail.
[snip]
I wasn't trying to bash roundcube, I'm using it as my main interface. I only user squirrelmail now when someone says: "It's prettier, is it faster?". I bet I'm not getting as much IMAP improvement since both of my setups access through localhost.
I'll try APC (from previous responder). Not counting that, does my idea have any merit? Regardless of how fast roundcube is, it seems loading message header while the user is busy looking at other messages can only help. I like to set my "rows per page" to 100 (or at least 50) and that takes time regardless of which front-end I'm using. Of course, it may not be possible to render a row then update later once the info comes back from IMAP.