DELL LATITUDE E6420 PRECOESSOR BEHAVIOR

Hello guys, I own a Dell Latitude E6420 and I run it on Windows 10 pro. I have being having this issue; When I use my laptop while the AC power is connected the processor speeds come down to a constant 0.60Ghz making my laptop slow but when I run it on battery without AC connected the processor speeds shoot up to 3.10 Ghz making it very fast and smooth.My BIOS version is the A23. I have tried and tried adjusting power plans but still no change. I kindly request any assistance to my issue from a good willing person to really sort me out. I have used the laptop now for 3 months. My System specs are;

Operating System: Windows 10 Pro 64-bit (10.0, Build 10240) (10240.th1.170602-2340)
Language: English (Regional Setting: English)
System Manufacturer: Dell Inc.
System Model: Latitude E6420
BIOS: Default System BIOS
Processor: Intel(R) Core(TM) i5-2540M CPU @ 2.60GHz (4 CPUs), ~2.6GHz
Memory: 4096MB RAM
Available OS Memory: 3978MB RAM
Page File: 5022MB used, 1340MB available
Windows Dir: C:\Windows
DirectX Version: 11.1
DX Setup Parameters: Not found
User DPI Setting: Using System DPI
System DPI Setting: 96 DPI (100 percent)
DWM DPI Scaling: Disabled
Miracast: Not Available
Microsoft Graphics Hybrid: Not Supported
DxDiag Version: 10.00.10240.16384 64bit Unicode

ANY ASSISTANCE I WILL WARMLY APPRECIATE, THANK YOU