I would really like to see some empirical evidence supporting this assertion.
In my personal experience and from a few blog posts I have read it is not very much 'lighter' than other 'main-stream' DE's
such as Cinnamon.
I'm not doubting the statement as fact - just wanting to know from a 'scientific' standpoint.
Is it 'truly' 'lighter'?
I guess that if one goes about it scientifically, you would have to download and install all the desktop environment or install various desktop environment on the same distribution.
I just have experience with Gnome, KDE and LXDE. The Gnome one was the heaviest on my system, KDE was lighter and LXDE is definitely the lightest.
I downloaded Lubuntu with LXDE because it was smaller with less programs I would not need anyway. I have a tricky 3G internet that work fast sometimes and is very slow other times.
I personally have found that a well compiled Linux distribution saves me a lot of time and trouble, as opposed to installing some base and then trying out, which programs work well with it for two weeks.
I tried a minimal install of Q40S based on Debian 9 and had a lot of trouble choosing the right music player and other programs to work without hick ups.
I have also read that XFCE and LXDE have less bugs than the prettier DE, like KDE. I have only one PC so I haven't tried out them all. Budgie looks like a good comprise between performance and good looks.
Personally I am sticking with Lubuntu because it works so reliably, other people might get bad flashbacks of Windows XP and find it too bland for their taste.