By what is my iOS OpenGL ES 2.0 app's performance bound?

I'm currently learning Objective-C. As my first project, I've chosen to write a small 3D app for iOS, using only OpenGL Es 2.0. So, yes, I'm learning that at the same time. I only knew the basics of old-fashioned OpenGL (without shaders) before, but I can do (and did implement) some basic Matrix and Vector math.

I'm making some progress, but the performance isn't that great anymore. I started profiling in Instruments (Xcode 3.2; I do have access to Xcode 4 through the iOS d开发者_开发百科ev program, but refrained from using that because of some strange behavior), but I can't quite make sense of the data:

The app (running on an iPhone 3GS, iOS 4.2) runs at about 15 fps, but Device Utilization is only around 40%, Renderer Utilization around 35%, and Tiler Utilization below 10%. What does this imply? Am I bus bound? I thought I'd need to hit near 100% utilization in at least one of those three statistics before seeing the fps drop (and yes, the app can hit 60 with very simple scenes).

At the moment, all the app is doing is render a scene graph containing 500 untextured cubes with normals, bunched into groups of 1 to 6.