LXDE was very light yes, but it was built on GTK+2 technology, which meant the moment users started using GTK+3 apps which most people would have done, some of it’s lightness was gone as both GTK+2 libs (used by desktop) and GTK+3 libs (used by modern GTK apps) were both in memory; memory was wasted having two libs that did the same thing in memory.
LXDE is dead, developers mostly jumped to RazorQt and it became LXQt. LXQt uses the Qt5, modern toolkit (found in KDE, android phones and more including apps found on windows).
The desktop LXDE wasn’t always light for end-users, as to remain light meant they had to consider their choice of apps carefully, especially where ram is limited (less than 4gb in my opinion, though this figure is subjective and depends on what you’re running).
(I personally found Lubuntu 18.10 & 19.04 faster (or lighter) on my single core pentium m dell latitude 610 & ibm thinkpad t43 than Lubuntu 18.04 LTS; on my dual core pentium iv hp dx6120 though I couldn’t decide)
Personally I’ve found LXQt to be equally light as LXDE, often better than LXDE in 18.04. The biggest hurdle to me was the switch from older GTK+2 to Qt; which in order to remain light requires you to switch to Qt apps (but GTK+2 apps were few anyway; and Qt apps will be mostly better than using GTK+3 apps on LXDE anyway).
This is opinion. My experiences will differ from yours because we will be using different apps. To me the amount of RAM a box has is very significant, as more libraries in memory can cause page-faults and thus paging to disk which really hits performance. Its application choices that keep this down, making the desktop choice a somewhat insignificant part of the ‘light’ equation (it’s a starting places yes! a significant one, but not the key point).
There will be end-use cases where Xubuntu may in fact be lighter; it’s now fully using GTK+3, so if you only used apps for GNOME it will very possibly perform better than a LXDE or LXQt desktop. Modern Lubuntu (with LXQt) is light when used with Qt based apps; but even that’s not everything. Pull in some KDE (and thus Qt) apps and you may discover lots of stuff gets pulled in, why? because some KDE apps are written expecting Plasma (not just Qt) making application choice messy again. When you install an app though, and you see what gets pulled in - this provides a huge clue as to how performance will be in my opinion.
My point is desktop choice is only the starting point. A light desktop with the wrong apps will use more resources than another [less-light] desktop using native, or apps using the same resources as the installed desktop.
The desktop developers, and Lubuntu packagers have and do consider this; thus
leafpad was replaced by
featherpad, my beloved
gpicview replaced by
lximage-qtetc. However if users don’t want to make the switch and continue to use apps they are used to, the ‘lightness’ of the desktop is gone.
Lubuntu is light in my opinion, but keeping a desktop light requires more knowledge from users, than they possess or want to know about - thus is wasted effort I fear.
I am an end-user, and I love my
gpicview, so for me I’ll often take the performance/lightness hit to use the picture viewer my fingers know. It’s my choice, and the efficient or light choice made by Lubuntu devs gets wasted on me. I do tend to decide when I use
gpicview, for if I’m using apps that are memory hungry I’ll use
Distribution/desktop choice is only a starting point.