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.
That may be true about the ~500MB RAM, I did not measure it or anything and I agree: if you want to go for <100MB RAM you'll need to look for something else. I actually used i3wm (with separate programs) before on my older computers, but I was pleasantly surprised by KDE performance and decided to stick with it.
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.