A couple of other things to check:
a) At least with the RTM Vista and Office 2007, I found substantial speed
improvement when the front end mdb was on the local computer instead of in
the user's documents on the server. Some of that difference may have
disappeared with Vista and Office service packs, but it can't hurt to try.
Vista was much more dependent on having recent drivers for the network
interface.
b) Check that .mdb's are excluded from anti-malware scanning everywhere: on
the local machine, the server and the network.