I have discovered that the Reat-time scanning module McAfee Anti-Virus Plus causes the Mozilla Thunderbird email program to run very slowly with the default settings.
Unchecking the "Email attachments" box in Real-Time Scanning Settings appears to solve the problem.
According to these posts it is/was supposed to have been fixed. Now whether or not that fix has gone out yet or not I don't know, plus it also depends on whether or not you get your software directly from McAfee or through a 3rd pasrty, such as your ISP or PC maker for instance as they often hang on to fixes for 'testing purposes' anhd sometimes don't release them at all.
Technical Support would be able to enlighten you there. It's free to phone or use online chat and linked under Useful Links at the top of this page.
I am running version 12.1.253 - in addition automatic updates are on, and the interface was update within the last two months by one of the updates.