Performance in build significantly lower than in-engine.

When I run a game I’m making in-engine I average about 60 FPS, but when playing it in a standalone build I seem to be getting what must be around 12 FPS.
Not sure why that would be happening. I did test on the “Fastest” graphics settings, and it made it a bit better, but marginally so.
Anyone have any ideas?

Could it be that in-studio, you are not rendering your game at full resolution?
In my setup the unity preview pane is more like one fourth of my total screen resolution.