Android performance problem on model joints(Rig).

Hi

Why dose device performance dropped after adding joint to model and skin them! (Rig the model)

My android device can render about 200 instance of my model (1000 Tri by Maya) with 60 fps,
but when I add joint to the model (just 3 simple joint) and skin them (simple skin), the device can render about 30 instance with 60 fps instead of 200!

What is the problem with joints on models?!

Anyone?

Check it yourself,
Make a cube with 10000 face in Maya and make a copy of cube and rig it(add joint and skin in Maya).
Then compare the performance of simple cube and rigged cube on an android device by unity!