I just contacted customer service from Apple. The guy one the phone said that he hadn't heard of the problem before, yeah right...
He told me to do a reset (cmd+option+P+R when booting).
I thought it had fixed the issue but after 15 minutes of (fluent!) work, the spinning ball appeared. Even when I just enable full screen mode the spinning ball appeared...
I was running Intel Power Gadget the whole time and the temperature was not over 60°C. No fans what so ever.
So my guess its just an incompatibility issue between either MacOS, FCPX or the new Intel Chipset.
Anyway, my return window ends the 22th of june, I think I will return this unit and wait for a solution.
I do not want to buy a 16inch because of the formfactor. I like the smaller size of the 13inch a lot.
It is supposed to handle even 4k. This is not acceptable.