Are you shooting in bursts? If so, what is D5 set to? That will limit the number of shots in your bursts.
Are you shooting Large TIFF? Those will fill up the buffer in the camera with about 23 shots. Once the buffer is full, even the fastest memory cards will slow you down a lot. Fast cards take up to 10 seconds to write RAW or TIFF images.
What kind of batteries are you using in your flash? Lithium are really slow, and can take up to 30 seconds to cycle the flash. Alkaline are slightly better. If your flash were firing at 1/4th power, you could shoot 4 shots in a burst before the flash would stop you to recycle, and then there would be a long wait with lithium or alkeline. For fast recycle time, use NiMH rechargeables.