I don't know what they did to it, but Roundcube 0.5 is incredibly fast. I run it for my private webmail; it's actually been broken for me for a while, logging me out a few seconds after I logged in. With 0.5 I re-configured it from scratch, the bug is gone, and it runs like a native mail client - loads even huge folders in less than a second, loads mails almost instantly...it's awesome. Nice to have working webmail again...