Man, I feel so left out of the party. I've reindexed my mail, tried compacting my folders... Still takes 11 seconds between the time I click on an email and for it to come up in the preview area. This is even true for pure text emails.
Sad part is, Googling for posts or pages regarding slow Thunderbird 3 either gives no results or shows errors early in beta that were fixed. I have yet to find someone that has my issue.
This is SO very annoying. It's now faster for me to check my email from my iPhone at my desk than it is for me to do so on my PC.