[Dovecot] Loading messages

Thanks to all for pointing out the Thunderbird bug workaround, I've got another question

Some of my users are complaining that when they select a message, the email client says "Loading Message" and either takes a long time to display a small message or does not display the message at all. Its odd becuase it seems that larger message on these users machines seem to open more quickly then the small 1Kb - 15Kb messages. Looking a little further on the messages that wont open they all seem to be very small in size. Is this a common issue? or something i need to tweak on the dovecot.conf or on the client side?

Gabe