I've got a couple old laptops from 2007/2011 era - are you talking about KDE Plasma or a different DE? I'm down to try just about whatever you recommend.

I run KDE Plasma on a 10 years old underpowered tablet (1G RAM, Intel Atom 1.66 GHz, slow SSD - 32G too) [1]. Anything else is unusable on this tablet. It is slow but it works. Xfce and LXDE are probably lighter, but were barely usable on it when I tried and I'm not sure they would be so much faster. Browsers and apps are certainly what eats your RAM, and with KDE you get apps that share a lot of code in memory. Xfce / LXDE don't provide many things so you will need some Gnome or KDE apps anyway.

I wouldn't bet that Xfce is that much lighter than Plasma nowadays (but definitively disable features like search indexing on slow hardware). On 2G of RAM, you probably should choose between KDE, LXDE, XFCE, maybe MATE (or lighter environments), the one you like the most. I can't say for Cinnamon ; probably fine. Gnome is probably too heavy.

[1] The Airis Kira Slimpad

Good deal. Thanks for the opinion.

The KDE team put a lot of work into memory optimization. It's more efficient with memory than any of the other lightweight ones like xfce.

I'm using plasma, the default for opensuse leap.