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.