Hi,
What we do know is that google uses allot (and really allot) of proprietary stuff, they even invented their own filesystem. On top of that they throw an enormous amount of hardware and caching agsint their mail systems. I totally doubt you could ever make a client faster then google. But who knows..
second question : google imap is always blazing fast i migrate allot of email off google through imap and it goes super fast. Never seen any trouble with imap. You can however optimize roundcube and imap as described in some docs. (caching : memcached, some other php modules,, imap proxy etc..) and most important good hardware (not implying anything g just summing up), but i am sure there are experts here far better then me that can explain that.