1. An image processing apparatus configured to rasterize an object into a bitmap using a first memory and a second memory which can be accessed quicker than the first memory, the image processing apparatus comprising: an extraction unit configured to extract a plurality of objects to be rasterized on the second memory from a plurality of the objects; a first combination unit configured to combine a plurality of objects which can be rasterized within capacity of the second memory from among the plurality of objects extracted by the extraction unit into a single object; and a rendering unit configured to render the single object on the second memory and render an object other than the plurality of objects extracted by the extraction unit on the first memory without using the second memory; wherein the first combination unit includes a determination unit configured to determine whether rasterizing time which is required to rasterize the combined objects on the second memory is shorter than a predetermined target time, and the first combination unit combines a plurality of objects whose rasterizing time is determined longer than the predetermined target time by the determination unit, into an object in a bounding box; wherein the image processing apparatus further comprises a determining unit configured to determine whether rasterizing time for rasterizing an object which cannot be combined by the first combination unit on the second memory is longer than the target time, wherein if the determining unit determines that the rasterizing time for rasterizing the object on the second memory is longer than the target time, the first combination unit designates the object as an object to be combined into a bounding box.