> KDE or XFCE seem to mimic the Windows 2000 design in many ways, but they are still far away from feeling as snappy or as well-thought out than Windows 2000 did. They also paradoxically feel more "gray" than I remember Windows, even though the "grayness" of Windows from that era is sort of famous.

I haven't used XFCE, but you can attribute the lack of snappiness in KDE compared to early Windows to compositing and having more animations. There's not much you can do about compositing, it's kind of necessary on high resolution computers, but Wayland latency has been getting better and if you use a recent distro like Fedora it feels about the same as Windows 7 with compositing enabled. For animations, you can speed them up or disable them entirely using the "global animation speed" slider in the settings. For the grayness, you can re-enable colored window titlebars in the settings by going to "Colors & themes" -> "Colors" and then selecting "Breeze Classic". I don't know why they have them disabled by default.