Slow FPS remains even after prim.dispose()

Hi,

Just to report that on the example given in the documentation:  http://www.babylonjs-playground.com/#OWCCR#8 

Increasing the number of primitive slows down the fps, but when reducing that number fps stays low even after dispose(). 

I don't know it if was reported before, and I did not remember this behavior last time I checked this example... @Nockawa