Hi Jaycompo,

Thanks for posting. Apologies that your system is not performing as expected.

Have you tried any of the methods shown in this post from the Forum?

If you need additional assistance, please contact me privately. Be sure to include your personal information and your computer's service tag number. Thanks.

Robert there's no solution until you give me a modified bios. it's a throttling issue, the laptop runs fine on battery so just tell me where can I get good bios that won't throttle my cpu or gpu.

ahhh, found what you are talking about with Bios.

You cant disable Intel Speedstep.

Try another charger and I bet it works.

Download Core Temp and report what CPU temp is while plugged in and throttling. 

Go into Windows Power Management and verify it is set for maximum cpu performance when plugged in.

Same with the graphics card setting you find there.

Get back to us. 

Googling it I am finding many found it was a bad Dell charger ...

http://www.tomshardware.com/answers/id-3215066/slow-cpu-speed-plugged.html

I have disabled speedstep, turbo boost and c states too from the bios but problem still persist

I have set power options to maximum and same with the graphics card, it's not windows problem coz I have reinstalled windows , it's probably that a sensor has gone bad and sending false bdprochot signals and cpu is underclocking while charging

If you need additional assistance, please contact me privately. Be sure to include your personal information and your computer's service tag number. Thanks.

Look at the plug from the charger that goes to laptop. That tiny center pin if bent or broken, the PC will think its an unknown charger and lower cpu. Something with the charger is out of Spec.

The internet is flooded with stories about this very same issue. All were the charger.