Speed difference between Agilent 82357B and NI GPIB-USB-HS

I have both the Agilent 82357B USB GPIB interface and the NI GPIB-USB-HS USB interface.
Transactions on the NI device are about 30% slower than the Agilent. (i.e if a bunch of commands take 150ms to complete on the NI device, they will do the same in 100ms with the Agilent)

Is this likely to be a driver difference or is the Agilent device doing a better job than the NI?

Michael