An image forming apparatus includes a data storage unit, a plurality of data processing units, and an assignment unit. The data storage unit is configured to store data of pages onto a plurality of storage devices. The plurality of data processing units are configured to perform parallel processing on the data of the pages, wherein the data for each of the pages is assigned to one of the plurality of data processing units. The assignment unit is configured to assign the data of each of the pages to be processed by one of the plurality of data processing units, wherein the assignment of the data of each of the pages to one of the plurality of data processing units is determined by a transfer rate of each of the storage devices for the data of the pages stored on the plurality of storage devices.