Vista - USB has 7us dead-time in Vista 64-bit downstream from Hubs!

We are testing our USB devices on Vista Home Premium 64-bit and we are seeing that USB Bulk IN Transfer timing within a microframe from the same endpoint/device when the device is downstream from a HS hub is radically different than on all other 32-bit Windows OS's or without the hub.

If ...
... there is a HS Hub between the PC and the device AND
... running Vista Home Premium 64-bit AND
... a single Bulk IN transfer of 64Kbytes has been requested resulting in fast back to back bulk IN transfers (not NAKed) of 512 bytes with no other traffic between transfers,
Then the intertransfer delays are 7.0us between the Ack of the previous Bulk transfer and the IN of the next Bulk transfer (or any other bus activity). This dead time cripples our system.

If ...
... we run the same device on Vista or XP 32-bit OS with or without a Hub OR
... we remove the HS Hub but keep it on Vista 64-bit
Then the intertransfer delays are minimal at 1.1us between the Ack of the previous Bulk transfer and the IN of the next Bulk transfer.

So my questions are:
1) How do I get the USB bus intertransfer gap (the time from when one transfer finishes (ACK) to the start of the next transfer (IN)) on Vista 64-bit with a Hub to be as fast as Non 64-bit Windows or without the hub?
2) Is this a bug in the OS?
3) What causes the USB bus to be idle for 7 times longer between transfers in this case than in the 32-bit or non-Hub case?