Laptop slows down significantly while charging

Hey guys! My latitude E5440 keeps on having this issue where immediately as the charger is plugged in the laptop CPU clock speed drops to 1.98ghz but when not plugged in it has a normal of 2.6ghz... the laptop does not have temperature issues and it runs on an i5 4310 which indicates that it runs at 2ghz... I don't know why it caps itself at that and refuses to go past that whole charging but it's bottlenecking the Nvidia gt720m gpu that it comes with... I've updated bios, checked power options still to no avail... Please is there anyway to stop this?