Dell Latitude E5520 :: Keystroke Lags / Doesn't Respond / Slow / Unresponsive

I have a Dell Latitude E5520 and lately it has been extremely slow, and characters on screen tend to lag far behind my keystrokes occasionally not appearing at all. Sometimes when I click on a button it will lag for several seconds and then go through, other times nothing will happen at all and I will have to click the button again and it works. Recently the laptop was taken on a plane and the in-flight wifi was used, that is the only thing done recently outside of the normal routine.

I have booted into safe mode and the lag still occurs. I have antivirus software installed and up to date.