Qt bug related to key repeat is fixed in Qt 5.13.
However, key repeat settings are still ignored
on Arch Linux running Qt 5.13 + Plasma 5.16.2.
STEPS TO REPRODUCE
1. start Wayland session
2. open keyboard kcm
3. change delay and rate settings
4. click "Apply" button
5. focus "Test area" field and hold any key to test the new settings
no change is noticeble while you test the new settings
key repeat settings work on Wayland
Operating System: Arch Linux
KDE Plasma Version: 5.16.2
KDE Frameworks Version: 5.59.0
Qt Version: 5.13
*** This bug has been marked as a duplicate of bug 408943 ***