Adding Audio Source creates lag?

I am making a game where a boat flows along a river. Along this river, bridges spawn. I have these bridges as prefabs in the resources folder, which are then randomly instantiated. These bridges have cars that drive across.
My problem is that when i try to add audio sources onto the cars, even when the bridge isn’t instantiated, the game lags considerably (in the editor). When i remove the audio source, it runs smoothly again. I have no idea why this is happening and it’s really annoying!

Any help is greatly appreciated. Thanks.

DERP: By clicking off of the prefab in the inspector it doesn’t lag. Not quite sure why, but hey…