Ctrl+two finger scrolling on touchpad is too sensitive (XI2)

When using the two-finger scrolling on the touchpad to control zooming, the events are way too sensitive. This does not happen with a real mouse wheel.

To reproduce:

1. Open any Qt program that reacts to Ctrl+Wheel scrolling for zooming, like Qt Creator or KF5 Konsole
2. Press Ctrl and use the touchpad to scroll
3. Try to reset to exactly the original size

This did not happen with Qt 5.4 (I believe, it could be an older version).

This behaviour comes from XI2. Using xinput test-xi2 to test touchpad motion shows the exactly same too-sensitive behaviour (events are reported at every slightest motion). This does not happen with the older mouse emulation that you can see by running xev.

Is there anything that Qt can do to make this motion less sensitive so that the functionality can be used at all?