why the refresh frame rate is slow?

[boyks]

I am doing a image processing project. The image processing is implementing using glsl and rendering to a offscreen frame buffer. And then the 2D texture is using the QuickItem.update() to update to the screen.
My glsl shader of the image processing algorithm takes about 20ms for each 800*600 frame. My problem is the frame rate of the application is about 20fps. I measure the time from end of the glsl processing to the next start of the processing (which I think is the time for display) and it takes about 30ms to display.

Can anyone tell me what is the problem? all the processing and update is in the same thread.