for you mac users that use Mail.app, i found this (through digg) that will help speed up the application.

Here is a brief instruction on what to do (copied from that site):

1. Quit Mail.

2. Open Terminal.

3. Type the following:

cd ~/Library/Mail
sqlite3 Envelope\ Index

An sqlite> prompt will appear.

At that prompt, type vacuum subjects;.

After a short delay, the prompt will return. Type Control-D to exit.

4. Restart Mail and enjoy the extra speed.

UPDATE:
apparently this works too…
sqlite3 ~/Library/Mail/Envelope Index vacuum index;