Empty scene @ 60fps vs City Scene @140fps... ?????

I'm not new to Unity or development, but this one is stumping me. I've been trying to optimize one city scene which had a lot of buildings in it, etc, so I was selectively hiding or altogether removing certain components (buildings, trees, other characters, items/pickups, etc) to try and figure out where I could make cuts to improve performance.

I wound up backing up the scene, and just deleting everything and running it. Still maxed out at 60fps (actually 58fps) running an empty scene with no scripts or anything. So I booted up another city scene with a smaller city, and ran it for comparison which ran at 140fps.

I'm trying to understand why an empty scene is running at less than half the framerate of something that actually has content, environment, characters, audio, scripts, HUD, etc.