4+ minutes to open Preferences (Mail 6.5)

As the title says, it takes more than 4 minutes to open Mail preferences window or to switch from the settings of one account to another one.

Other than that, the app works correctly, I can retrieve new emails, compose a new one, forward, reply, print… and response time is totally normal.

It's running on a Mac Pro under OSX 10.8.4. Any idea about why and how to fix this?

Thanks!