Yeah I remember when there weren't dozens of services running in the background just for basic OS functionality.

What do you miss the most? The UI? Speed?

UX, speed, simplicity, lightness. Applications ran without talking to the internet, asking if it's ok to run it, telling me it's probably unsafe to run it, telling me I need to update for security reason, telling me I should play candy crush, letting me search my files without adding recommended noise somewhat supposed to be relevant to what I did 3 days ago. I could go on. I just want to stare at a blue flat colour knowing tomorrow it will be just the same. /s

Yes! UI was amazing and obviously you can’t beat the speed it would run at on modern hardware

Honestly I think Xfce is about the same. And probably more stable, though obviously it's hard to do a direct comparison.

Depending on the "Distro" and compile options/build flags/included libraries MATE is as fast and light, since at least 6 years, actually. I think the first distribution which showed that to a general audience was LMDE a.k.a. Linux Mint - Debian Edition.

The window chrome is fine but the settings are a bit of a mess in my opinion.

UI shouldn't be too hard (https://www.wincustomize.com/explore/windowblinds/8628/). I am not so sure about the speed if you'll use modern drivers.