Mapman - async USB puts the timing and request under the control of the external device, not under the control of the PC. The USB device is optimized for that and does not have other functions to distract it. So, all the PC and player have to do is keep the buffer full, which it should be able to do without much problem. Other than that, the timing is controlled by the external device.
Here is an explanation of how async USB works, from the guy who introduced it.
Async USB
Here is an explanation of how async USB works, from the guy who introduced it.
Async USB