For example, you can achieve this by configuring your DNS. As fas as we understand, your users are scattered across multiple sites, and the slow connections between them make the Web interface slow if users get connected to a Web interface located not in their own site. In such a case, you can configure the DNS servers in the different sites so that the Web Interface address would be resolved into different IPs depending on which site a user comes from. Thus, in each site, the Web interface address needs to be resolved into the IP of the 'local' Web interface.
That's, probably, the only option we can come up with.