What causes "The speed of processor 0 in group 0 is being limited by system firmware" in the event log?

It's a Windows bug caused by Intel DPTF or Intel CPPC in new machines. It's caused by throttling of the CPU by the Intel software.

You can try putting the power-plan to performance and also try setting min and max processor states to 100%.

Just adding this here, because I had this problem, and after trying all possible solutions, including BIOS settings and setting the min and max processor states to 100%, it turned out I had something really silly.

My Samsung Laptop (NP900X4C), an old Series 9 laptop with Intel i5, but still going strong, suddenly started to throttle itself to 47% at 0.80Ghz. I was also getting the strange behaviour that upon boot up it first goes to 100% until all the services are started and then it throttles itself with that "The speed of processor 0 in group 0 is being limited by system firmware." event.

After trying all solutions I found online and nothing works, it turned out that my laptop has this 'silent mode' which I could enable by Fn + F11, which slows down the fan speed but also throttles the CPU. I had never used this mode, and I must have hit it by mistake. Silly problem, but if you don't know about it will make you go round in circles.

Kernel-Processor-Power Event ID 37 is not an error, it is a warning. It informs you that your system runs on low battery and therefore has reduced the speed.