You may this problem as I did, when you delete a message in Thunderbird you have to wait even a few seconds to see them disappear. Of course in the meantime Thunderbird becomes unusable, you cannot open a folder or anything.

I thought this might be a folder size problem, perhaps I had too many messages, but even after I removed almost everything from the Inbox the slow delete problem remained!

Then I thought that my Gmail account had some configuration that caused problems, I checked and found nothing.

The slow delete problem was solved at last when I emptied the Trash folder. It seems that when I deleted a message Thunderbird needed a lot of time to copy it to the Trash because it was full of messages resulting to a large archive that Thunderbird had difficulties to handle. Now that the Trash is empty, Inbox messages are deleted instantly as before.