My Dell 5510 DE takes 93 seconds to run compared to 24 second on a 3 year old 4600U based laptop.
sysbench --test=cpu --cpu-max-prime=20000 run
I'm running kernel 4.4.0-78, BIOS 1.20, disabled Intel SpeedStep in the BIOS just in case.
I have noticed that the CPU speed is 800000 and scaling governor was powersave even when the device was connected to a Dell 130W 6.67A power adapter.
Update: Upgraded to BIOS 1.2.25 and tested with Live Ubuntu. Still no luck!
Seems that this is a known issue plaguing the XPS line.
Unscrewed the back cover, disconnected the battery for several minutes and everything is back to normal.