I'd say the difference between dead and stable is if you encounter bugs all the time. I find Thunderbird stable (only triggers a bug roughly once per month and nothing critical) while the Lightning plugin for calender functionality is dead (fails to sync with Google for months).
FWIW, I do use Lightning all the time and only know of a couple of minor UI bugs, and at least one other person in my house uses it to sync with external calendars routinely without serious problems as well, so if there are serious bugs there then they don't seem to be universal. In any case, I'd much rather fixing this sort of thing was the priority for the project than yet another UI theme overhaul because reasons.
The order in which you install add-ons, such as the provider for Google calendar and Exchange calendar support, matters. I've spent too long jiggering with add-ons to get them to play nicely together.
Once I have a functioning setup,I copy the profile wholesale from machine to machine. I use Thunderbird every day and have done for years.
I consider that a local problem that only you have. None of the several person using TB daily for more than a decade have encountered this kind of performance issue.
What have you tried to find the cause of your performance issue ? Have you tried making a new user profile to test if your local profile got somehow corrupted ?
Yes, I've tried many things, including a new profile. Compacting would help for a few days. Thunderbird just doesn't seem to do well with large mail archives going back more than a decade. That is my experience and the experience of others.
Also, I've met many people who experience similar performance issues, so I find it bothersome that because you've never encountered it you're asserting it does not exist.
This is also an issue that's span multiple machines and multiple clean installs.
Add me to your new list then. TB starts up in ~1min. Sometimes it will just hang for a few minutes while updating emails/calendar. (Can't switch to its window) Sometimes it will take 10s+ to dismiss a meeting reminder. Sometimes the meeting reminder can't be dismissed. Sometimes it will get confused by filters working slowly and both beep for a new message and then immediately move it away.
There are bugs upstream for all of those. They're up there for years.
