This invention determines a compression method of printing data in consideration of a printing operation halt time when a low-speed interface is used. An image processing apparatus of this invention, which is connected to a printing apparatus, compresses printing data, and outputs the compressed printing data, determines the interface type, acquires a print waiting time, and determines the compression method of printing data based on the determined interface type and the acquired print waiting time. The image processing apparatus compresses printing data using the determined compression method, and outputs the compressed printing data to the printing apparatus.