In previous versions of KDE the Pointer Acceleration option available in System Settings-->Input-->Mouse could be used to control the speed of the on-screen pointer relative to the movement of the input device. Using Plasma 5 I have not noticed any change in mouse input after changing this value, even between absurd values (0.1x --> 11.0x). Reproducible: Always
After more testing, this appears to be an issue with the use of libinput on Fedora 22. Uninstalling libinput causes this configuration option to work.
*** This bug has been marked as a duplicate of bug 350688 ***
*** This bug has been marked as a duplicate of bug 383379 ***