This has been an issue with Parallels for as long as I can remember (and continues to be quite unacceptable). This is what I used to do to at least make it somewhat usable:
via: https://forum.parallels.com/threads/ability-to-disable-smooth-scrolling.350866/
To disable smooth scrolling manually, please shutdown the VM > locate in Finder > right-click and select "Show Package Contents" > open config.pvs in TextEdit > locate this key and change the value to 0:
<SmoothScrolling dyn_lists="">
<Enabled>10</Enabled>
</SmoothScrolling>
Then press Cmd+S to save the file > close it and start the VM.
You can also mess around with Xorg configs a bit to manually change the scrolling speed. KDE provides a nice settings panel to do this from. The real fix would be to fix your code @parallels.