What?? It ain't about threading as there are no threading issues with VFP and IIS. Performance under light to medium load should be pretty much equivalent to ASP.NET

As long as you compiled a multi-threaded dll and not a single threaded dll, you should have no problem with multiple users from anywhere internally or externally. Also check Instancing: in Project Info and under Servers, make sure it says "Multi Use".

Also make sure you're using the Session class and ComReturnError for errors.