I notice the Mail application on my Mountain Lion has been running slower and slower. Lately, it’s so slow that the spinning wheel comes out and stays for a while from time to time.
It turns out there is a way to speed up Mail. It’s to rebuild the email database.
Just go to the Mail menu and click Mailbox, then click Rebuild.
Then I suggest you go take a walk. There is really no spinning wheel of any sort to notify you what’s going on, in fact you may not even notice if anything is happening in the background. But from my experience, after you come back, Ctrl-Q your Mail application. Then open it up again, Mail will start indexing your emails. After a while depending on your mailbox size, your mailbox will be a lot faster than before.