Thunderbird runs very slowly periodically

Recently, Thunderbird will periodically become almost completely unresponsive. If I type while composing, the keyboard buffer will write at a character per second or so, The system will often grey out the page (Ubuntu's way of saying that the application has hung).

The activity manager shows nothing happening. This might go on for a minute or several, and then the system reverts to normal and I have the Thunderbird I love.

This *seems* to have started since I removed lightning and enabled the inbuilt calendar extension. Could be a coincidence, but I've been using TB for well over 10 years and this has never happened before. I have two calendars, the inbuilt one with almost nothing in and a very heavily used Google one with lots of repeat events. The Google one is read only (annoying), and I mainly use the extension to be able to read invites and add them to my calendar and then copy them into Google (manually).

I have compacted my folders. If there was a stack dump utility that let me see what was going on "under the hood" I think this would be easy to solve.

I am running 60.2.1 (64-bit) on Ubuntu 18.04 with the old Unity interface.

Thanks for any suggestions!!