Is there a mix of color and B&W pages in the document? If so, most machines will pause to change the fusing temperature, image quality, ect. There should be a "productivity mode" you could turn on that might help alleviate this, but if there is a lot of mixed mono and full color then there's not much you can do.
On our 1000i, it will pause a lot when a job with mixed mono and full color will start, but once it gets warmed up and running, 4-5 booklets, it seems to run at a pretty consistent speed.