Because i've had the same problems with a application working with data on a WAN I applied the following:

Where ever it is possible, try to use some kind of replication of remote data to a local network drive. This can be done with data that doesn't change to often or devolop a good replication mechanism.

Use as much advantage of Rushmore. The gain of performance by using rushmore is tremendous. I found factors of 1: 1.000.000 (on very large tables) and more.

It is more efficient to download a lot of data and use it for many modules than to download data every time you enter another module.

I hope my suggestions will help.

Good luck.