Not rebuilding all, rebuilding packages that did not get an update or rebuild for a .so change.
KaOS is a rather fast rolling distribution, packages that do not get an update/rebuild after some time end up being build on much older versions/libraries then are currently in the repositories. One of the reasons it all runs so smooth is exactly because all is build so tightly integrated.
You certainly will start noticing a slow-down, if some of the very low-level (core-main) packages will not be rebuild in 2 years or so.
Plus by making sure every package is checked at least once a year, any upstream change has a much bigger change of being seen.