Why do engines run faster if you only have one line of analysis open? When I load up an engine (like Stockfish), it reaches a higher depth quicker if I only leave one line of analysis open. If I leave multiple lines open, it runs slower. Why is this?
That's normal, commonly known as multiPV analysis.

Without getting into too much technical details, analysing N number of principal variations require running the engine N times, one for each PV. We need to do that so we can estimate a precise score for each PV.