I'm not sure I understand the point of your tests at all. OpenGL will make no difference whatsoever to the time it takes to render an image.

Also by the look at your figures above something tells me that your scene was entirely empty of everything except for a sky. Is that correct ?

If so, that's not a real world test at all and you wouldn't have any clue of the true impact the OpenGL 2.1 places on your resources.

Regards
James