mate is definitely lightweight. Many people find it’s old-school aesthetics off-putting.
However, if you like it, it is both lightweight and stable.
That being said, I still think the best solution is to use the DE you like the best and tweak it’s resource constraints to meet you needs.
KDE to make it more lightweight can turn off all “desktop effects”. Have to untick them all one by one as no global untick button.
Compositor. Can turn that off. Or perhaps compromise test try Xrender so it uses CPU instead of GPU but you still lose some effects. This is if your GPU is too old, but your CPU is decent.
Use Show FPS in desktop effects to check consistent 60FPS performance
Everything relative.
Xfce is still lightweight; the proprietary DEs take what, 2-3 Gb. < 700Mb for a fully operational DE is still light as heck.
Xfce is also noticeable snappier than both Gnome and Plasma. It responds quicker.
I don’t know Deepin’s numbers but as far as I know the heaviest DE at the moment is Cinnamon, being built on a version of Mutter that isn’t the latest. So for Mint users Xfce is definitely a more lightweight option than Cinnamon.
Also, of course, there is the truth that “Unused RAM is wasted RAM” and a very low footprint at boot looks cool on screenshots, but unless you actually have any issues what matters is how the system behaves after hours of uptime and proper usage.
Stil would go for lxqt 
kde has annoying bugs that xfce doesn’t have. for example, I just used kde for a few months and had the annoying bug of copying something and it wouldn’t copy, and I would have to copy it 2-3 times for it to work. it seems like a non-issue until it happens every single time for months and becomes extremely annoying. xfce seems more polished to me
KDE is most of the time ok, without issues. And it is like fully customizable. Also very high development.
Xfce is good, smooth. I saw about their development like every few months. And it is not lightweight as before. Almost same as KDE
but yeah, I like both 
XFCE with Endeavour OS theme just pure awesomeness. My favourite distro theme.
XFCE is very light and agreed that it takes around 700mb on first boot which again is very manageable.
I don’t like the direction gnome is headed. It seems they are prioritizing form over functions. It’s bare bones right of the box but if you still need extensions to make it function and basic common extensions then why not just included it. Also, if the extensions break over time then what do you do?
Also, for me personally I don’t care about tweaking or getting extensions. I am passed that stage. I just need something fast and stable just works out of the box which XFCE offers and happy with it.
Xfce is still lightweight in my estimation. My Arch install with Xfce boots to about a 315Mb ram usage. Sure, If I start some software, especially a browser, the ram jumps up. But 315Mb at a fresh boot is lightweight.
It is always the question, How does it use the ram… not about how much…if you move some stuf to the cache you also dont see how it actually uses… its also contradict a bit but stil gtk3 has also its thing…
most importand how desktop feels and responsive.
Most complain about Xfce 
but stil there is a bunch of distro’s that uses Xfce i think more distro’s actually… its only pitty Xfce does not get too much love
wow how ? xD
If your computer is less than 10 years old, any desktop environment will work without any problems.
Except GNOME, of course, which is always problematic, but not because of resource usage. 
Xfce is really good, but I switched to i3 and sometimes KDE because xfce is not good with games on Steam.
Dont know why. even Gnome behaves better
I personally have gone all around with most of the DE’s. Gnome was my go-to when I started using Linux. it’s way before GNoME went nuts. After gnome dropped trying to be gnome. I tried KDE, LX (whatever), Cinamon, Elementary, and MATE.
I didn’t like any of them until I got to XFCE. It’s minimal functional not much of a looker but if the user can make it look better if he/she spend some time. If I didn’t use XFCE I would have used KDE or Cinamon. XFCE is light but once we loaded with some nice eye candy it does lose the lightness but is still better than most (for me).
But anyway I personally don’t worry about RAM or other stuff my PC can handle itself and most PCs nowadays are able to handle Linux DE’s. Except for Gnome I guess.
I had to switch to XFCE recently because plasma shell was using a lot of RAM after some time I use plasma . So I guess XFCE is still light weight for me 
Been running XFCE almost exclusively for 4 years, first with Mint then EOS. Never had a problem with Steam.
Everything is light weight. . . Unless you need a web browser. In which case all DEs are heavy in 2021.
I use my own bash script installer, which just does the steps from the Arch wiki in a script.
For most new computers whether it uses 300 MB or more is insignificant! Xfce is very light compared to most desktops. I have no memory issues on any desktops no matter what it’s using.
As I have just switched from KDE to Gnome and to XFCE just last night (all fresh installs), my current experience on how much ram they use is like this:
KDE = XFCE < Gnome
Gnome used 8GB out of my 16GB after some use, gaming, web browsing etc
I have seen KDE and XFCE are along 2-3 GB use. Honestly would have gone back to KDE after all the installations but after trying XFCE i like Thunar more than Dolphin.
PS: Only gripe I had with XFCE is it doesn’t have a built-in nightcolor changer, got around it by installing Redshift.
Gnome used 8GB, interesting. I’m using Gnome and have Firefox open with 20 tabs pinned & 3 tabs open and I’m at according to htop 3.6GB. Like what @ricklinux said back in November of 2021 I have no issues either, is the 8GB on a fresh boot? 