Poor playtest performance in Unity on Mac after switching platform to iOS

I've started a very simple demo in Unity 5.3.2 running on Mac OS X El Capitan. The project only uses the standard airplane asset and a terrain created using the asset "Low Poly Terrain." It play tested fine in Unity, then I built it for iOS. Switching platforms took a long time, but despite throwing a bunch of warnings, it worked great on my iPhone 6.

However, when I went back into Unity to continue making changes and playtesting them, the performance of the game pane in Unity was unuseable- like one frame every few seconds,

After switching the platform back to PC, Mac & Linux Standalone, the playtest performance went back to normal.

Since going back and forth between platforms takes so long, this makes developing for iOS impossible.

Any ideas?

Thanks!