I have Plasma running on an old ThinkPad I5 4th gen. with 8GB RAM and on an old HP tower with an I7 third gen. with 6 GB RAM and as @Kresimir already wrote, Plasma runs reasonably smooth on both of them.
Granted, they’re not my workhorse machines, and in comparison with my two Ryzen machines, well let’s not make that comparison because that is not fair competition…
The big slowdown factor is the browser and surfing the internet in general, but the DE itself does perform reasonably well on these machines.
I also want to stress that running a DE/WM is one thing, maintaining a DE/WM for an ISO with an installer on board is a completely different ballgame.
We as a team love this project and we would also love to maintain it for years to come. As @joekamprad and @dalto already mentioned, Plasma out of the box does fit better to lighten the workload and that is what it is all about.