USB has one stage more than PCI or PCIe
the Audiodriver have to wait for the OS-USB controller/driver.
Audioprogramm --> Audiodriver --> USB-Controller (WIN/MAC DRIVER) --> DA-Converter
Audioprogramm --> Audiodriver --> North?? or Southbridge?? --> DA-Converter
so.. in fakt the Mainboard North or Southbridge is much much faster in latency than USB.
sure it is portable .. but you never get 8 channels with 1,5ms ..
3ms not even be possible..