XPS 9550 Always Slow Performance, again the power settings BUG?

Hi,

I have a Dell XPS 15 9550 with the following configuration
16 GB Ram
500GB SSD
Full HD Screen

I experienced a big loss on performance from saturday. I did nothing different from usual, this is the PC that I use for work, so just normal operation with or without the wired power supply.
I noticed that because everything I do now is slowed down, from browsing the web to start programs, to run also 3D Applications.
The PC is completely updated to all the new Windows 10 updates and drivers. I did drivers updates through the DELL tools.

From the Task manager perfomrance monitor I noticed that the CPU speed is always 0.78 Ghz instead of 2.6 Ghz, no matter if I run heavy aplications or I plug or unplug the power cable.
I read from this forum several topics about a BIOS bug on this PC: entering the power save mode and going back to normal mode unplugging and pluggig the power cable, doesn't really affect the PC to go back to the full performance settings. People found a temporary solution logging off or restarting the PC, or disabling the SPEED STEP and C-STATE option from the BIOS performance panel. A DELL offical employee followed the case and DELL updated the BIOS tagging the solution as as solved.
I updated the BIOS yesterday to the last one : 1.2.18. --> Problem not solved
I disabled the SPEED STEP and C-STATE option from the BIOS --> From the Task Manager Performance monitor i can see now the speed of the CPU back to the full speed but the PC is still slow exactly as before.
I tried to switch between different power settings, from HIGH PERFORMANCE to BALANCED to DELL without any success or effective changes on the performance of the PC.

Do you know any solutions? I really dont know what to do now to have back the full performance of this PC.

Regards,

Matteo