Mail very slow in Mountain Lion

Question

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

Answer 1

This didn't solve my problem. First of all:
     $ ls /Library/Preferences/com.apple.mail.plist
     ls: /Library/Preferences/com.apple.mail.plist: No such file or directory
but I do have the file in my local Library:
     $ pwd
     /Users/spate/Library/Containers/com.apple.mail/Data/Library/Preferences
     $  ls -l *mail*
     lrwxr-xr-x  1 spate  spate     64 Jul 26 11:08 com.apple.mail.LSSharedFileList.plist@ ->      ../../../../../Preferences/com.apple.mail.LSSharedFileList.plist
     -rw-------  1 spate  spate   6733 Jul 27 08:24 com.apple.mail.plist
     -rw-------  1 spate  spate  56130 Jul 27 08:12 com.apple.mail.plist.bak
This is after I moved com.apple.mail.plist to com.apple.mail.plist.bak and then started Mail again. As you can see, it created a new, much smaller com.apple.mail.plist
No luck - just as painfully slow as before!
This is horribly painful!