> XFCE GTK engine is much faster than Adwaita
Not to rain on anyone's parade but the XFCE GTK engines haven't worked in a heck of a long time because theme engines were removed ("deprecated", quoth the GTK team, as in they left the interface in but removed the actual functionality, so they didn't do anything anymore) many years ago. The default XFCE theme (Greybird, I think?) uses CSS, just like Adwaita, and there's basically no difference between the two.
Everything else is true as far as I know :-).
Thunar is much faster than Nautilus, XFWM is faster than Mutter, XFCE panels are much faster than the Gnome Shell, XFCE GTK engine is much faster than Adwaita... and the list just goes on and on. The result is a really fast and responsive user experience, that will run fast even in low spec machines.
I am not a fan of the default panel layout and overall configuration, but XFCE panels are easy to configure as well as highly configurable. You can create as many panels you want, position them anywhere you want and put any elements on them... There's even a way to create a macOS-like global menu, although I don't recommend it.