Bottlneck in fractal animation?

Tiny program “Mandelbrot set zoom”
http://www.ibiblio.org/e-notes/webgl/webgl.htm
uses fragment shaders to make fractal animation (600x600 pixels and only in one point yet :slight_smile: On Radeon HD 3650 (120 shaders 750MHz) RivaTuner shows me GPU load only 30%. I wonder where is the rest 70% ?