I used to get this a lot but haven't seen it in a year or two. But I have switched to the message-per file storage format, so that may be part of the issue.

I have 366k messages, but all in one folder and used to see hangs even when that folder wasn't open (but it was being synchronized).

I still get hitches and sometimes crashes due to not responding for too long when navigating other things, but message composition seems fine now.