What is claimed is:1. An image forming apparatus comprising:a printer configured to perform printing processing by an electrophotographic method; anda controller having one or more processors which executes instructions stored in one or more memories, the controller being configured to control the printer, wherein the controller:decompresses compressed quantized raster image data in units of pages;derives an image feature quantity from decompressed raster image data in units of pages;notifies the printer of the derived image feature quantity; andtransfers the decompressed raster image data to the printer in units of pages in response to an image transfer request from the printer, and the printer:sets a parameter relating to a fixing process in the printing processing based on the notified image feature quantity; andperforms the printing processing in accordance with the set parameter for the decompressed raster image data received from the controller.2. The image forming apparatus according to claim 1, whereinthe controller performs derivation of the image feature quantity of a preceding page and the decompression of the next page of the preceding page in parallel in a case of performing the printing processing continuously for a plurality of pages.3. The image forming apparatus according to claim 2, whereinthe printer sets the parameter of the preceding page based on the image feature quantity of the preceding page and the image feature quantity of the following page thereof in a case of performing the printing processing continuously for a plurality of pages.4. The image forming apparatus according to claim 1, whereinthe image feature quantity is one of a toner application amount, a coverage, and an average print ratio.5. The image forming apparatus according to claim 4, whereinthe parameter relating to the fixing process is a fixing temperature, andthe larger the toner application amount is, or the higher the coverage and the average print ratio are, the higher the fixing temperature the controller sets.6. The image forming apparatus according to claim 4, whereinthe parameter relating to the fixing process is an interval between sheets at a time of sheet conveyance, andthe larger the toner application amount is, or the higher the coverage and the average print ratio are, the wider the interval between sheets the controller sets.7. The image forming apparatus according to claim 1, comprising:an interface that receives target data of the printing processing from an external device, whereinthe compressed raster image data is compressed raster image data received from the external device or data obtained by compressing raster image data generated by interpreting PDL data received from the external device.8. The image forming apparatus according to claim 1, whereinthe controller performs derivation of the image feature quantity after performing predetermined image processing for the decompressed raster image data.9. The image forming apparatus according to claim 8, whereinthe predetermined image processing is one of processing to rotate, move, or scale the decompressed raster image data in units of pages and processing to combine the decompressed raster image data of a plurality of pages.10. A control method of an image forming apparatus comprising a printer configured to perform printing processing by an electrophotographic method and a controller configured to control the printer, the control method comprising, by the controller:decompressing compressed quantized raster image data in units of pages;deriving an image feature quantity from decompressed raster image data in units of pages;notifying the printer of the derived image feature quantity; andtransferring the decompressed raster image data to the printer in units of pages in response to an image transfer request from the printer, wherein the method further comprises, by the printer:setting a parameter relating to a fixing process in the printing processing based on the notified image feature quantity; andperforming the printing processing in accordance with the set parameter for the decompressed raster image data received from the controller.11. A non-transitory computer readable storage medium storing a program for causing a computer to perform a control method of an image forming apparatus comprising a printer configured to perform printing processing by an electrophotographic method and a controller configured to control the printer, the control method comprising, by the-controller:decompressing compressed quantized raster image data in units of pages;deriving an image feature quantity from decompressed raster image data in units of pages;notifying the printer of the derived image feature quantity; andtransferring the decompressed raster image data to the printer in units of pages in response to an image transfer request from the printer, wherein the method further comprises, by the printer;setting a parameter relating to a fixing process in the printing processing based on the notified image feature quantity; andperforming the printing processing in accordance with the set parameter for the decompressed raster image data received from the controller.12. A control method of an image forming apparatus having a memory for storing decompressed data, comprising:decompressing compressed quantized raster image data in units of pages;storing the decompressed raster image data in the memory;deriving an image feature quantity in units of pages based on the decompressed raster image data stored in the memory;setting a parameter relating to a fixing process in printing processing based on the derived image feature quantity; andperforming, in accordance with the set parameter, the printing processing of the decompressed raster image data in units of pages stored in the memory.13. The control method of the image forming apparatus according to claim 12, whereina memory for storing the decompressed data used for the printing processing and a memory for storing the decompressed data used for deriving the image feature quantity are a same memory.