I have similar feelings to everything you just said.
I assume the appeal to LXQt isn't so much that it's "lightweight" as that it's "simple". Because, like you said, Plasma is actually very lightweight and snappy, and I'd be willing to bet there are very few places where LXQt will run well and Plasma won't.
Not in my experience, though I have pretty high/less realistic standards coming from i3.
It's plenty usable on modern hardware (anything midrange+ from the past 10 years or so), but it is worth noting that I've still run across folks running core 2 duos in recent times. (Although fwiw they were running plasma, just complaining about it not being very responsive)
KDE can be configured to run with pretty minimal overhead, but it takes quite a bit of knowledge about how the DE works. After disabling the fancy compositor effects though, I think I'm comfortable saying that it's more responsive than my i3wm setup on a 13-year-old x201. Not a knock against i3, as it's a much better tiling WM than KDE ever will be, but I really think the resource requirements aren't the far off from one another.
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.
>[..]In my experience, they'll tend to have worse vsync issues and tearing.
Thankfully I've been able to avoid that for the most part, but I will say it's certainly easier to avoid with plasma's compositor.
>I doubt i3 is actually drawing shit on the screen faster than Plasma with animations off.
I'd definitely be interested in a raw speed comparison if there was a good method/benchmark, but I don't think that has ever been my limitation (although if I were running on lower spec hardware everything counts)
Plasma certainly has more features, but it's still definitely heavier than i3 (or most (all?) tilingWMs), LXQt, icewm, and the like.
On decent hardware there's not a huge point other than preference, but when you've got 2GB of RAM and a 12 year old CPU every bit helps https://i.imgur.com/vZszQFE.png
In the case of a simple window manager like i3 I cannot imagine what it would need to do in a second thread and without a compositor it would definitely be drawing faster if not perceptively faster. I don't think one would be apt to notice any difference in performance save for plasmas still slow startup.
I could I suppose point a camera or a phone at the screen and measure I suppose.
What I'm most interested in is the latency (I guess that's more or less the same as "snappy"). Back when I had a netbook LXDE was one of the few that felt good.
There's certainly some added latency in GPU composited desktops so a lot of the time that has an effect with modern desktop environments and/or wayland vs LXDE which as far as I know doesn't run a compositor.
I'd love to find some good testing on it somewhere, but I do recall off the top of my head that Windows XP or Windows Vista/7 (with Areo disabled) had significantly lower latency than the GPU accelerated counterparts in Vista+. I believe it has to do a lot with Vsync, but imagine there's some additional overhead involved as well.
Of course many would say removing screen tearing is a valid tradeoff there, I mostly agree but not everything is significantly effected by tearing either.
I assume the appeal to LXQt isn't so much that it's "lightweight" as that it's "simple". Because, like you said, Plasma is actually very lightweight and snappy, and I'd be willing to bet there are very few places where LXQt will run well and Plasma won't.