I'm running Plasma on a low-range laptop from 2010 and it runs exactly as smoothly as it does on modern hardware. It was $600 in 2010, has 4 GB RAM and one of the earliest generations of the Core i3, with integrated graphics.

If your acquaintances have machines that struggle to run Plasma, I'm honestly surprised they can run graphical apps written in recent GTK or Qt versions at all.

In many cases, Plasma's going to run better/smoother than these "lightweight" window managers and desktops. Because, all the Stockholm Syndrome-ing about "simplicity" often translates to "probably is single threaded and does more CPU work than it should because it's easier." In my experience, they'll tend to have worse vsync issues and tearing.

I doubt i3 is actually drawing shit on the screen faster than Plasma with animations off.