I saw a breakdown of how the X11 protocol over a network works (IIRC It was by one of the wayland proponents who also was a an xorg maintainer so he knew what he was talking about).
The protocol was very poorly designed everything needed essentially several trips back and forth between the client and server. That applies also if you're using Athena or motif widgets, it just becomes much worse with QT or gtk because they also transmit bitmaps, because otherwise they can't guarantee the look.
So it's latency not bandwidth which kills the performance. If you need 3 or 4 roundtrips just moving a window around, that's fine on your home network where you have a ping of 10ms, over the Internet with 100ms latency 3 roundtrips become 300ms so just a cursor move becomes painful.
The protocol was very poorly designed everything needed essentially several trips back and forth between the client and server. That applies also if you're using Athena or motif widgets, it just becomes much worse with QT or gtk because they also transmit bitmaps, because otherwise they can't guarantee the look.
So it's latency not bandwidth which kills the performance. If you need 3 or 4 roundtrips just moving a window around, that's fine on your home network where you have a ping of 10ms, over the Internet with 100ms latency 3 roundtrips become 300ms so just a cursor move becomes painful.