If local discovery is enabled or you enter the IP:Port of the other Node in the device settings, instead of dynamic, then syncthing will only transfer locally. You can check that by expanding the device on the right side of the web interface. If it shows a local IP, than it is syncing locally.
I did set up the sync between my desktop and server with their local IP set in the device settings of each other, so they can only sync locally, and I have the same low speed as you have.