High CPU usage since December, how can I check what is causing it?

Yes there are a couple of ways but they are not easy to interpret.

Profiler Scan (this will work better after 2024.2 is released with Python 3.12 support as it will show the thread info). But is still worth trying now:

Also Py-Spy.

Run the scan and ask for help interpreting the results in this topic if you need it: