Seems like a very misleading chart IMO. Maybe they have a disclaimer that omits RME devices from the results? ;-)
AFAIK, The UCX @ 64 Samples @ 96K (as the chart mentions) over USB would be 64 In + 64 Out + 32 (safety) + 14 (ADC) + 7 (DAC) = 181 Samples @ 96K. This breaks down to less than 2ms (# of samples / samplerate = 1.88ms). Otherwords - on par with the HD Native PCIe system!
Maybe Protools is adding some latency of its own to the mix when using a USB device? I have no clue...
Regardless - Something smells very wrong with that chart!
MADIface-XT+ARC / 3x HDSP MADI / ADI648
2x SSL Alphalink MADI AX
2x Multiface / 2x Digiface /2x ADI8