Having successfully convinced me to move away from Xfce after GNOME 2 was deprecated, my main DE has been MATE for such a long time. However, I am being wooed by KDE Plasma lately. I remember running Plasma 5.26 on Slackware 15-current and was blown away at how snappy it was on an old Dell Latitude E6410 with a 1st-gen Core i5 520M! I can only imagine how nice Plasma 6.x is in comparison.
MATE has also been stable for me on the BSD side, running it on OpenBSD and FreeBSD, but Plasma might woo me away on there as well, especially once Plasma 6 is available on OpenBSD 7.6.
I also prefer to run Fluxbox on much less powerful hardware, regardless of the OS it’s on.
I can speak from experience that it is worth it. It won’t be a super speed demon, but it will make it somewhat more usable. I’ve done so with my Asus Eee PC 901 netbook which has the two PATA SSDs. Those SSDs are SUPER slow compared to the cheapest mSATA SSD you can find with more than double the space, and all you need is a MiniPCI-to-mSATA adapter (the Eee PC 901’s drive slots are MiniPCI). I documented all about it here: https://claudiomiranda.wordpress.com/2020/10/04/my-geeeky-experiment-part-3/
I’m running OpenBSD/i386 on mine which isn’t as fast as something like Linux, but it definitely felt faster even with OpenBSD after the hardware upgrade. I also increased the RAM to 2 GB which is the maximum amount supported.