Intel Visual BIOS has power saving settings that you can adjust. However, the processor is a 15 watt rated processor. See procrssor specifications here: Intel® Core™ i5-6260U Processor (4M Cache, up to 2.90 GHz) Specifications
The NUC power requirement changes depending on the workload. The maximum TDP is 15W and according to the Configurable TDP-down technology of the Intel® Core™ i5-6260U it can decrease up to 9.5W.
Now, it is possible to customize the power settings of the NUC using the operating system too. By default, Windows® changes dynamically the power settings of the NUC.
If your NUC is running at maximum TDP, I suggest you to verify the speed of the memories in the BIOS. They should be working at 2133MHz at 1.25v. The Processor Base Frequency is 1.80GHz.
Intel recently released a newer BIOS version 0028 available in the link below.
Thanks for your answers.
I have checked the BIOS and everything seems normal.
So Windows can set the frequency to a value lower than 1.8 GHz?
Also, I find it quite strange that the frequency goes up to 2.9 GHz even under light loads (< 25% in the task manager).
I would rather let it run at 800 MHz or something like this when the load is unter 50%. Is this possible?
Well, when you reduce the clock speed, overheating is impossible since clock speed scales linearly with power dissipation. But anyhow, it looks like there is no way to achieve what I was looking for.
At least, I have now disabled TurboBoost in the BIOS and now the Core runs much cooler (~50°C). I think the NUC ist just not designed for 2.9 GHz, it gets very hot and loud at times.