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.
Hi, 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. Help! Update: Upgraded to BIOS 1.2.25 and tested with Live Ubuntu. Still no luck! 
Dell 5510 DE single core is nearly 4 times slower then on a three year old 
laptop when running sysbench