fans always at max speed on xps 15 with Ubuntu linux

since the installation I was not happy about the fan management on my Ubuntu 18: the temperatures were always too high despite the fans were running at more than 4000 RPM (often even more than 5000).

After several months where all my attempts went from bad to worse,
I ended up on this page

 so, long story short:

1. launch the GUI nvidia-settings  
2. select the Prime Profiles
3. switch to Intel GPU
4. then I was requested to reboot but I preferred also to add another change suggested at this other post
5. I created as root the file /etc/modprobe.d/nvidia-user-registry.conf with just a single entry
   
   

   options nvidia NVreg_RegistryDwords="OverrideMaxPerf=0x2"
   

6. and finally
   
   

   $ sudo update-initramfs -u
   $ sudo shutdown -r now

after the reboot the system is 100% fine - the fan speed is almost always less than 3000 RPM and the temperatures all below 55.0°C