Improving Apple Mail performance
There is a solution (found via Hawk Wings) which involves a bit of Terminal usage that will boost your Apple Mail, and it will reduce the used diskspace as well. The speediness was noticeable in my case, and diskspace was reduced at about 5Mo.
First of all, quit Mail, open a Terminal, and change the directory to Library/Mail:
daniel:~ daniel$ cd Library/Mail
Then, open the Mail sqlite3 database issuing the following command:
daniel:~/Library/Mail daniel$ sqlite3 Envelope Index
Rebuild the subjects database and exit:
SQLite version 3.1.3
Enter ".help" for instructions
sqlite> vacuum subjects;
You may open Mail now and see if it has improved the performance for you.
For those of you afraid of deleting your mails, here is what the vacuum command does: http://www.sqlite.org/lang_vacuum.html