Kernel task takes about 500% CPU after connecting external monitor
I would suggest moving your charging plug from the left side of the MacBook Pro to the right side. Similarly you can experiment with plugging in the monitor on the other side.
The two sides have different reactions in terms of temperature, so on one side it might not make much difference - and on the other side it could be just above the threshold that triggers an attempt to lower the temperature by making kernel_task occupy the CPU (in idle mode) to save power.