I use Mail.app with a .Mac account, 5 Gmail accounts, and a local ISP account I got years ago. Over time, Mail.app seems to slow down as it deals with these massive indices. This app seems to compress the mail database, but it saves me a trip to the Terminal to do so.

I have observed the 5 Gmail accounts take some time to display the messages in the Inbox immediately after opening Mail.app (in the order of 3-5 seconds) where the .Mac messages appear right away. After running SpeedMail, the Gmail messages appeared right along with the .Mac message after opening Mail.app. In other words, no delay.