Slow mail response with constant beach ball.

For the last month I experience a slowness (spinning beach ball) for about 1 minute when I wake the computer and mail is already open. It shows well in excess of 100% cpu activity but it may only download 3 messages with little data. Has anyone also experienced this problem. I have been on Support with a senior tech and after reinstalling mail, cleaning up the number of emails I still experience the same problem. I am running on the newest iMac and have very few programs open on the desktop.