DrawPixmap works differently. It draws the individual pixels to the OpenGL buffer in the correct format. I don't think there should be any speed difference if you store it in a different format.
If you can use TImage instead of TPixmap, do it as it will be much faster. If you need your image to change a lot, I think you'd be better off writing directly to texture memory instead of to a pixmap.
If you can use TImage instead of TPixmap, do it as it will be much faster. If you need your image to change a lot, I think you'd be better off writing directly to texture memory instead of to a pixmap.