CPU running slower than it's base clock speed when on battery

Intel Speedstep technology takes advantage of a power-management technique known as demand-based switching (DBS) and is designed to dynamically modify the voltage of the CPU—and hence the processor clock  speed—based on CPU load. As a result, DBS enables processors to generate less heat by operating at the minimum voltage and clock speed necessary to perform the required operations. For example, on the Intel Xeon processor, reducing power consumption from 1.4 volts (3.6 GHz) to 1.2 volts (2.8 GHz) can reduce the heat generated by over 30 W.  So if you disable this feature you may face issues like system overheating. You can also make disable this feature in BIOS and check if it makes any difference. 

Steps to enter system BIOS:

• Restart the computer.
• As soon as you see the blue Dell logo start tapping the ‘F2’ key.
• If you wait too long and the Microsoft Windows appears, then continue to wait until you see the Windows desktop. Now shutdown your computer and try again.
• After this BIOS would appear . You can only use the arrow keys of the keyboard to navigate between the options as the mouse is not functional in this mode.
• Navigate to advanced tab and disable "Intel (R) SpeedStep (tm)".
• Press F10 and restart.