Tim Gaden of Hawk Wings fame (and formerly of APC X-Factor fame, sniffle) has documented an excellent tip to speed up Apple Mail.

It involves optimising the SQLite database Apple Mail uses to store indexes and subject lines of emails.

You can check your current 'envelope archive' size by entering this in the terminal:

ls -lah ~/Library/Mail/Envelope\ Index

Then to optimise it (cleans out stuff that has been marked for deletion but not actually deleted, defragments the structure, etc):

sqlite3 ~/Library/Mail/Envelope\ Index vacuum;

Then check your envelope archive size again to see the results…

ls -lah ~/Library/Mail/Envelope\ Index

It compacted my envelope archive down from 55MB to 50MB — not a huge increase, but the speed difference was dramatic.

In particular, my 'sent mail' folder which had been taking 10 - 15 seconds to open (8,000 items) now only takes two or three seconds.

Read the full post and many informative comments here.