Microsoft made significant improvements in performance of Internet Message Access Protocol 4 in Exchange 2010, and also emphasize importance of knowing the mailbox size when sizing your Client Access Servers for IMAP4 users. “Comparison with Exchange 2007 – Exchange 2010 reduces total CAS CPU/user by 40% while reducing memory footprint by 30%. These results were obtained with 100 MB mailboxes filled MAPI-format messages, typical of enterprise environments dominated by Outlook users. Not reflected in these results are improvements made to further reduce CPU costs with non-MAPI (MIME) messages. In the comparison below, users are doing same set of actions, but E14 system was able to respond more quickly to requests so number of concurrent connections dropped even while the fetch rate increased. We used IMAP4 fetch commands to normalize our results - the 'fetch' represents a unit of work which includes, for e.g., a portion of logon & logoff actions. The results here’re for a stand-alone CAS server (with 2-duel core Xeon Processors at 2.33GHz) in a multi machine topology,” explains Microsoft.
More info: IMAP4 and Exchange 2010 RTM