honestly its probably due to the cpu load caused by the 3rd party driver
i realize they were the ones to code the driver for this ESI company from the start..
but realize that other drivers for solutions would be very much different ie: PCI, Firewire
this is probably the core problem with why so many mac users were skeptical about usb interfaces back then
usb audio interfaces were looked at as a joke for a good long time as they were introduced..
perhaps you have stumbled upon that very reason behind that old belief.
this is totally not the case for windows/macosx probably due to improved usb performance
and it may not be the usb speed itself but rather that the system is using some cpu to facilitate the usb
thats why firewire was always looked upon as better then usb on the mac AND the pc because
usb requires cpu to regulate the function of it.. whereas firewire + pci do not require this extra cpu to manage the datathruput
http://www.diffen.com/difference/FireWire_vs_USBTypical USB PC-hosts rarely exceed sustained transfers of 280 Mbit/s, with 240 Mbit/s being more typical. This is due to USB's reliance on the host-processor to manage low-level USB protocol, whereas FireWire delegates the same tasks to the interface hardware (requiring less or no CPU usage).
this quote is referencing a pc.. but its the same on mac, usb requires cpu in its function, firewire does not.
the improvement u are experiencing is likely because the pci card maybe has some chips on it that are regulating the usb on the card itself? im not sure.. its a guess.