[RCU] Large inbox slow

We are hosting a webmail service using roundcube 0.8.1 and a number of our users with inboxes in excess of 8000-10000 messages are experiencing extremely slow response times (We also downloaded a recent copy and it's just as slow). We are using Dovecot 2.1 as the IMAP server and have not had any issues with clients like outlook or Thunderbird, but as far as we understand it those clients keep a local copy of the messages for quick retrieval while Roundcube is doing everything over IMAP. Is there any thing we can do or should look at to improve the response times for loading an inbox with many thousands of messages?