Yes the server has a few tweaks to speed up conversation loading and some popular queries(is:unread). We are always trying to find ways to cache and speed up the
JS code. So you'll see fixes to that in most releases.
NIO removes the limitation that the current IMAP has which every connection takes it's own thread. Using NIO we now can handle multiple connections in a single thread. So for huge deployments 100's or 1000's of users will see much better memory usage (ie less). If you've only got a 10's of user's you may not see a huge difference. Of course the fact that IMAP IDLE is now back on.
NOTE: It seems under certain cases the new code will still lock-up with IMAP IDLE. We are looking at that now.