I just did some experiment yesterday.
On debian, lxde-qt now eats 109 MB of RAM after a cold boot.
On the same machine, the old lxde gtk+ version only used 85 MB.
(On archlinux, it uses about 150 - 210 MB depending on the machine.)
The difference is quite obvious. :-(
Though 109 MB is good, I think there's room to make it better.
The most memory hungry programs are pcmanfm-qt, lxqt-panel, and lxqt-runner.
I already tried to optimize pcmanfm-qt earlier but it's not possible to squeeze much from it since the wallpaper is a huge bitmap and must eat some RAM.
Other parts are hard to optimize as well. No obvious memory eater was found during the profiling.
Much RAM was used by the icon pixmap cache, but it's inevitable. Otherwise we'll get slow painting.
The other parts used most RAM were lxqt-panel, and then lxqt-runner.
I tried to put them together in the same binary, and tested it again.
If we merge lxqt-panel + lxqt-runner into a single process, the used RAM becomes 103 MB after cold boot on the same machine.
The drawback of this approach is, we cannot have an independent lxqt-runner program.
A simple way to fix this is making lxqt-runner a module - liblxqt-runner, and let lxqt-panel load it.
When a standalone lxqt-runner program is wanted, we can have a simple program lxqt-runner which loads the lib. Both lxqt-panel and lxqt-runner share the same liblxqt-runner.
The drawback of this approach is also obvious. To make it a library, the compiled code will be PIC, which is more inefficient and less optimized.
I'm not sure if this will really have impact on perceived performance.
Some more experiment needs to be done for it.

Any comments?