Macbook startup problem. Once started is extremely slow I have a serious problem with my Macbook pro retina 13" early 2015.

Some days ago, after a normal shutdown, the Macbook did not startup anymore. It emitted the short start sound and it went back to off state again. Yesterday, I realized the macbook is able to startup correctly only if the power button is held down for a long time (until the apple's logo appears). Nevertheless, the boot up takes long time and the system is extremely slow. From the activity monitor, I see that "kernel_task" (PID 0) is wasting lot of cpu.

Actually, the system is not overloaded since the battery lasts for many hours and the case is not heated.

I tried to update macOS to High Sierra but an error occurred while verifying firmware.

Hardware diagnostic, temperatures, fan seem to be ok.
I tried to reset the SMC and PRAM but nothing changed.
In safe mode is still slow.
The problem remains after erasing the hard disk and reinstalling El Capitan or Yosemite.

EDIT: 

I tried this.
http://blog.kosmokaryote.org/2012/12/technology-kerneltask-consumes-almost.html

It works for me regarding the kernel_task problem.

Anyway, it is clear there is an hardware issue.
Most likely a hardware defect. Does it run hot as well?

Try running Apple Diagnostics, this might give you an indication something is wrong indeed. Perhaps the fan died, or maybe one of the temperature or current sensors.

An SMC reset might be worth a try in case it is only the SMC no longer working properly.