Hi Rolf,
I just took a second look at your design and saw the FET temperature was actually not that high. Upon further inspection I realized its actually Max duty cycle that's limiting your max frequency.
the LM25117 has a min high side off time of 320ns that limits the Max dutycycle of the circuit. Since your min voltage is close to your output voltage this ends up limiting the frequency that can be used.
Please see figure 31, page 20, of the datasheet for more info.
Best regards,
Tommy