The application was developed (elsewhere) in Visual FoxPro 7 and 9. It runs locally, but accesses data on the server for some operations; when it does this, there are huge delays if the application is running in Windows 7, but not if it's running in XP Mode on the same machine. Any ideas? The developers are stumped.
That's possible for many reasons.
Firstly, on XP you have less traffic-intensive services running, for example you have IPv4 on XP, while IPv4/6 on Windows 7. Another may be Homegroup polling on Windows 7.
The likely reason, though, is better driver support for your NIC on XP.
Or this could rather be too much support in the Windows 7 driver. I would check the Windows 7 NIC and maybe try to disable "Large Send Offload" and/or "Checksum Offloading", if they apply.
You could also disable ipv6 in Windows 7 if you don't need it.