An image forming apparatus and a control method thereof configured to input PDL data and render the PDL data into image data to form an image, wherein a rendering speed in rendering the PDL data into the image data and an image forming speed are changeable, and by changing the rendering speed and the image forming speed within a predetermined power consumption, suppresses degradation in image forming speed. When an amount of rendered and stored image data yet to be used for image forming equals or exceeds a first threshold, the speed of an image data rendering process is reduced while image forming speed is increased. Conversely, when the amount of rendered and stored image data yet to be used for image forming equals or falls below a second threshold, the speed of an image data rendering process is increased while image forming speed is reduced.