How many shots are fired and at what power level before the slowdown begins? Is the power level in remote mode the same as in the on-camera mode? Are you perhaps running into the thermal overload feature when SB900 is in remote mode? Check the thermometer symbol on the back of the unit when it slows down.
I'm guessing here, but cannot come up with any other logical explanation.
HBB in Phoenix, Arizona Nikonian Team Member
Photography is a journey with no conceivable destination.