> Default lock screen experience still has a needless delay of 5 seconds when entering a wrong (even blank wrong) password, even on the first attempt.

I suspect that is not KDE's fault (or Wayland's) - it's probably PAM, which by default has a 2 second delay (+/- 50%). That default is extremely difficult to change, but you can configure it. See my instructions here: https://github.com/linux-pam/linux-pam/issues/778#issuecomme...

Also if you follow that issue you can see I've been trying to convince the PAM developers to fix it (by changing it to a 0.5 second delay, which is much more tolerable and no less secure). Unfortunately they have this weird idea that users want the delay, because it lets them recompose their thoughts after getting the password wrong or something like that.