thanks for your tip to check reddit.
There are many users reporting the same problem, whereby it is always assumed that 1 core is used to capacity and / or the maximum allowable cpu temperature leads to a throttling.
From this follows for me - it makes no sense to buy an 8-core high-end-PC at the moment, because the basic programming of the MSFS still comes from the 80s. DX 12 seems to use sometimes only up to 2 cores at the moment. It would probably be better to take a 4-core processor with an extremely high clock rate or buy MSFS in 5-10 years.