In my experience, KDE is not the bottleneck (all the DEs use pretty similar resources, give or take 300mb). Rather, the distro under it is what dictates performance, and that matters a lot. Some are slick and efficient, some are not, even with the same DE. For really minimal and 32bit hardware I would look at Puppy. But I run PCLinuxOS/KDE on a 2006 laptop (first-generation x64, 2GB RAM and spinning rust) and it’s not slick, but it’s usable. On my 2013 laptop (early i5, 4GB RAM, spinning rust), even Fedora/KDE runs well enough, and upgrading to 16GB RAM made it downright lively. Upgrade RAM whenever you can, find 'em an SSD, and pretty much any x64 system will perform adequately for basic use.