What is the antivirus and its settings for email? Run Windows in safe mode to test for interfering startup apps.

Thanks for replying. I tried turning off Real-Time Scanning in McAfee and it didn't help. Then I tried running Windows in safe mode and that didn't help - I gave up waiting for an email to open after waiting over 2 minutes. This is so frustrating - this all started happening in the last month

I don't think you want to turn off McAfee Real-Time scanning, but the mail scanning options should be disabled:

https://wiki.mozilla.org/Thunderbird:Testing:Antivirus_Related_Performance_Issues#McAfee

But if the problem remains in Windows safe mode, look through the list of items here to see if any of them apply to your setup.