XHCI Mode = Disabled
The on-board USB 3.0 port function like a 2.0 port
XHCI Mode = Enabled
The on-board USB 3.0 port function like a 3.0 port
XHCI Mode = Auto
The on-board USB 3.0 port function like a 2.0 port before OS USB 3.0 driver load.
If you reboot the OS, the on-board USB 3.0 port again function like a 2.0 port during this reboot BIOS phase before OS USB 3.0 driver load.
XHCI Mode = Smart Auto
The on-board USB 3.0 port function like a 2.0 port before OS USB 3.0 driver load.
If you reboot the OS, during this reboot BIOS phase, BIOS is "Smart" enough to avoid downgrade the USB 3.0 port back to 2.0 functionality before OS USB 3.0 driver load. So Smart Auto is faster than Auto on 2nd boot onward, but Enabled is fastest once you are sure the OS has the USB 3.0 driver installed, because it avoid the switching. Making the on-board USB 3.0 port function like a 2.0 port is mainly to support OS installation or to support OS that does not have build-in USB 3.0 driver, so that the USB keyboard would still work if the user plug-in a USB keyboard or any other USB devices into the USB 3.0 ports before the OS is installed with the USB 3.0 driver come with the motherboard.