Running any kind of Qt-5 application over a remote X11 connection seems to be very very slow. Constantly redrawing the screen.
Test with running a konsole over the line and in that run nethogs and you will see a constant trafic of about 1MB/s - For updating a few characters!!
In QT4 you could set QT_GRAPHICSSYSTEM to native to fix this, but this doesn't seem to work in QT5 any more. Basically makes QT5 unusable in a normal X11 environment.