Apple Mail, uses sqlite as the data storage for all the messages you send and receive in Apple Mail. After awhile, this data base gets blotted. When messages are deleted, the database doesn’t really shrink in size. So, it is good to clean it every now and then.
How to Speed Up Apple Mail
1. Open up Terminal
2. Enter the following command:
/usr/bin/sqlite3 ~/Library/Mail/Envelope\ Index vacuum3. Hit enter to run the command