I have similar issues. It is quite frustrating. All I can say, is that your UP speed matters. If you can get that to be faster, it will be helpful. I am still waiting for further illumination on this problem.
If you'd run Filemaker Pro on a remote desktop server in the same local network as your filemaker server, you'd be able to use remote-desktop on any OS to connect to the rdp-server. The only thing running over the internet is your screen-changes.
The advantages are:
1) You can use just one computer for multiple users at a time
2) Any OS (even linux) can be used
3) The rdp-protocol is fairly quick (on a windows rdp-server though) so even with a relatively low bandwidth it run quickly.
1) You will need a special SBA- or a VLA-licence for Filemaker Pro to be able to serve more than one user at a time and of coarse youneed to set up a rdp-server.
2) You need to set up VPN or SSHD to make secure connections, i would not recommend direct RDP, although that works fine, it's not safe.
3) If you only have one or two remote users, this is a relatively expensive solution.