Ok, well. Came to the solution by trying something else. In the example it was just not that clear cause of the 8 cores of the CPU. The renderthread should run on max. possible speed, but it is just one thread. The thread is just passed from core to core, so everyone gets a share.
But why does the CPU always pass the thread around and does not keep it in one thread? This is just a thought, but it may be that the CPU always tries to have the same amount of load at all cores because of heat dissipation. Metal deforms if heat is not applied equally over the whole body. If the area of heat production is larger, it is also easier to pass it to the cooling unit. Might also reduce power consumption, which also reduce heat production.
Thanks everyone for trying to help, always appreciated.