The Intel® Core™ i7-6700K has 4 Cores, Core 1 works faster always than the rest and the temperature is going to be higher too.
Cores speed is dynamic and they usually run at different speed. I suggest to verify processor speed and memory speed in the BIOS of the motherboard.
The Processor Base Frequency is 4.0GHz and
Memories should be: DDR4-1866/2133 at 1.25v
or: DDR3L-1333/1600 at 1.35V
Then, I suggest you to run Intel® Processor Diagnostic Tool and review the results.