I have only one object in the scene and it is lit with a directional light. It uses the mobile/diffuse shader. The issue is that on iPhone 4 the FPS drops from ~42FPS to ~11FPS when the directional light is enabled. Rendering is set to forward rendering and there are no other light sources. Shadows are not enabled.
Scene stats give a total of five draw calls, 3k tris and 2.6k verts. Having a single directional light should not have that kind of effect on FPS. What could cause this FPS drop?