I’ve been running Plasma on my Intel Skylake I5-6?00U with 8gb RAM since 2015 and its utterly fine, no sluggishness in plasma itself. It’s still using X11 though, on an old Kubuntu LTS.
Plasma isn’t THAT heavy that it should be expected to feel sluggish on that kind of hardware. And contrary to popular belief Plasma isn’t actually that heavy of a DE in terms of resources.
OP might need to try different compositor backends? I remember years ago testing each before settling on whatever gave me smoothest performance (maybe OpenGL3?). Actually I’m not sure if this is even a setting anymore in modern Plasma, or in Wayland