Q: Mail very slow in Mountain Lion

The upgrade to Mountain Lion went smoothly but Mail works really badly afterwards.

• When sending an email, it takes a few minutes before the mail goes out. And that's with any size message.
• Auto-completion of mail addresses while composing takes multiple minutes. Even after it finally comes up with a list of email addresses, pressing enter on one also takes minutes to acknowledge
• At various times, hitting the mail icon on the dock has no effect. The same is true with Command+TAB

Overall, the unresponsiveness is painful and making me wish I hadn't upgraded