Unity terrain very low FPS on mobile

I have tried everything to get the FPS up on my game. I have a test scene which only has a unity terrain object in it. On my Nexus 7 (2013) tablet I get an average of 20FPS.

I have made the game very ugly by setting the textures to 256 max size, pixel error on the terrain is set to 40, there are no trees or details (using 4 textures though). The terrain is also only 150x150 so it's not huge.

The quality settings are also set to "fast". If the terrain is removed on an existing scene which is using it then I see a dramatic increase in FPS (from ~20 - ~60).

Is there anything more I could do to try and improve this performance? Or should I get rid of terrain and try a different method (3d model)?