All,
The prior problems I have seen with plymouthd taking a continual 6% of CPU with kdm/kde3 on all prior Leap releases (see post of 5/7/18), seems to be fixed for the Leap 15 build. I have not stripped plymouth from the default leap 15 install, and I've now loaded kde3 and configured kdm3 as the displaymanager and at least so far on the virtualbox VM I have it running on -- everything is fine.
I'll let you know if it resurfaces.