Another quirk I was not aware of at the time: the GUI drawing CPU usage.
If you have a slow single-thread CPU and/or are using Windows, this will affect you. Workaround: minimize or close the Tixati window when not needed (do not just obstruct it by other windows)
See these threads:
(my in-depth reply on page 3 yet another surprise of Windows about mouse cursor)
This is possibly the reason you peak at 700 mbit/s