Is caching on or off?
of course, you deserve a better explanation.....
There are many things that can contribute to the program being slow. The two largest are the IMAP connection and the DB connection. If you're connecting to an off-host IMAP server, it tends to slow down. If the DB connection is off-host, that too can slow it down.
Basically, if you haven't looked under the hood, RC is a mix between a database application, and an IMAP client. With any IMAP client, things can slow down depending upon speed of IMAP server, current users connected, resources being shared elsewhere and other things. The IMAP server is typically the culprit, if you don't have caching enabled.
With caching enabled, the DB connection is typically the culprit in slowing the program down. Each time you move, a DB query is executed and the result returned. The speed can fluctuate depending upon number of concurrent connections being used, speed of DB server, shared resources elsewhere, open but idle connections. These things can contribute to it being slow.
So it's not necessarily the code that's being slow (although in this case it may be) but rather everything joined together can make it seem slower than normal.