What driver is faster -- PCI board or USB unit?

Currently, PCI board is our fastest option. The speed of the USB DAC unit is limited by the restrictions of the USB bus, which updates the values once per 1 ms (all 40 channels are updated at once). For the PCI boards, every channel is updated sequentially, with 1 µs delay (measured value; the data sheet for the chip used on the boards says 5 µs).