I noticed not long after upgrading to Mountain Lion that using the mail app was much slower. Simply deleting emails were taking about five seconds each. Every time a new email was selected, there would be a delay while the new email loaded.
Of course, this is not normal and dealing with a couple of hundred emails a day made it seem almost unusable. Fortunately, there’s an easy fix.
How to Fix Slow Mail on a Mac
Step 1. Quit the Mail application.
Step 2. Back up all data if you don’t have a recent backup. This is to ensure you can restore your system to the state it’s in now if this doesn’t work for you. Keeping backups via a Time Capsule or a separate hard drive is always a good idea.
Step 3. Open Finder and select Go-> Go to Folder from the menu bar.
Step 4. Copy the text below and paste it into the box that opens, then click Go.
Step 5. Delete the selected file titled “Envelope Index”.
Note: Deleting this file will move it to your Trash Bin. You should be able to remove it from the Trash Bin if necessary. You can also duplicate the file to create an extra copy.
Step 6. Open your Mail app. It should re-index your mailboxes, you may get a few pop-ups indicating that your mailboxes are being repaired.
After your mailboxes have been re-indexed, you should see a noticeable improvement in your Mail app. We cannot guarantee that this will work for everyone, but it has worked for myself and several others.