convert the artwork to cmyk, turn off the color correction, lower your output res to half of what you printed the first time. printing in the printers natural colors and by passing the rgb conversion lets you print at a lower output res than RGB conversion.
think of it this way, if the artwork is RGB or CMYK, the printer is using the same ink, if you can send over the file in cmyk and get the output desired, ie color match, then you are going to speed up the process simply because you can print at lower res output which is simply faster.
the trick then becomes designing in CMYK and converting photographic- raster work in cmyk. 99.9% of the time i get the cmyk to the correct output.
i spent years perfecting my proccess to my printers to get this to work, it does, and your machine can print faster.
sure you will run into a raster file that you get from somebody that just doesn't convert the way you want it to, but enough fc'n around with this philosophy and you get there....
designers of the world, listen, design everthing in cmyk and if its black use c=45 y=45 m=45 k=100 and the world will be a better place...