The first is the amount of working memory on the device (128megs versus 1 gig of ram)
The second is the amount of processing needing to be done (pc does all the work or the printer of converting print job to laser code)
The third is the processing power/type of processor doing this work.
The fourth is the size of data needing to be processed
The fifth would be the amount of image quality control that the printer does to ensure the stability of the print quality
The sixth (and last for this post) is the type of paper (the thicker the slower).