Extreme lag on play

I was messing around earlier when I realized I was only getting 15-20 FPS. I had a scene with maybe 50k tri’s in it. I deleted everything so I only had a single cube to stand on and I was still only getting 30 fps. This is very strange because in a scene with 1.5 million tri’s I can easily get 70 fps. Help???
I was using unity standard character controller and a single cube. No light sources either.

You need to use the Profiler to understand performance issues of your game: Unity Connect

Exactly the same problem, profiler shows nothing that can lead to lag problem. I disabled everything in my scene, except camera and low poly terrain I stand on.
This lag appears randomly, without activating or deactivating something. Sometimes I get ~60 fps, and suddenly ~6fps. I think there is a problem with Unity 2017.4.0f1. I am downloading latest version to see if it happens again.

unity gfx.wait for presets in profiler takes most of the resources, but sometimes I don’t see this process and it still lags, so I assume it is not that.

Sometimes when it lags like that Unity crashes.