The slowness is common as more and more messages get added into Apple Mail, and can be fixed by doing the following:

1. If you have Apple’s Mail application open, please close it (a simple Command + Q will do the trick).
2. Click on Spotlight in the Upper Right.
3. Type “Terminal”, you should see the Application “Terminal” become the Top Hit.
4. If Terminal is highlighted, press Return.  If not, select it by using your arrows, and press Return.
5. You should see the Terminal application start, and be dumped at a textual screen prompt.
6. At that prompt, simply type: “mdimport ~/Library/Mail” and press Return.
7. Depending on how much e-mail you have in your folders, it may take a couple of minutes as it reindexes your messages.
8. Once it’s done, load Mail again, and it should be speedier!

If you have any questions, please feel free to Contact Me .  Enjoy!