I've used KDE on a Thinkpad T60, it's about 17 years old, has 3GB of RAM, and a Core Duo. It ran surprisingly well. Replacing the HDD with a SSD can also make a noticeable difference, so you should consider that if you haven't already. I also turned off a lot of the animations and effects for better performance.
@boredsquirrel
While tedious, those effects can all be turned off in *"Workspace Effects" or whatever it's called. Not at my laptop to check. @Mint_Raccoon
The only effects relevant for performance are blur and background contrast. Turn those off if you feel the system is slow, maybe increase the animation speed and you're done