Not up on the internals of device drivers for USB drives, but generally speaking, I don't recall that they were that fast.
The fastest device you can use for a database is a local disk. It uses a 32-bit data path (if not 64-bit) for a single DMA transfer.
When you have a network device or a very slow storage device, remember that you have to pull in disk buffers through slower interfaces. Both are inherently SERIAL versions of whatever you are sending. (USB = universal serial bus; Ethernet is also based on a serial physical layer.)
Access needs to load up its internal data, then the relevant tabledefs, forms, etc. across that interface. Obviously, the faster the I/F, the faster you get response. Depending on complexity, I've seen speed increases of 15- to 30-fold depending on network vs local database rates. (NOT percent. A truly measured 30-fold speed difference).
Part of that might be due to one of the L2 or L3 caches between local disks and memory that don't apply to the network paths. But network devices, because they tend to work in smaller gulps and over slower transmission paths than disk devices, are just slower.
Remember that for networks, you are only as fast as your slowest hop.