Check how many long running, recurring appointments you have.

> high CPU usage until I refresh the calendar.

By this you mean, you start Thunderbird and it has high CPU. Until you click a calendar (you only have one calendar?) and pick "synchronize?

I have no recurring appointments.

Until you click a calendar (you only have one calendar?) - Yes after startup

pick "synchronize? - not an issue

The problem still exists. When I start Thunderbird the CPU usage goes to 12-15% until I switch to the calendar then is drops to 0.0-0.2%. The same thing use to happen with older version of this program that was eventually solved.

See https://support.mozilla.org/en-US/kb/profiling-thunderbird-performance for instructions on create a profile.

Here is the result of the profiler.

https://profiler.firefox.com/public/acnqgrm5bndv34czg9z6t915xya4v5b0yfr1q7r/calltree/?globalTrackOrder=0w2&hiddenGlobalTracks=1&hiddenLocalTracksByPid=4420-0w2~7824-2w5&thread=0&v=10

Thanks for the profile.

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