I heard KDE still uses ~500MB RAM on idle, which is too much for older computers. I don't care for animations and hiDPI but i would care to have <100MB RAM usage and 0% CPU usage on idle.

EDIT: Also catering to slow hardware is not just about memory usage, it's also about how much you need to read from disk. In this regard, both GNOME and KDE are pretty bad when running from HDD.

EDIT2: For comparison, Haiku OS says 384MB RAM is the minimum to run the entire system with desktop environment.