Not sure if this is relevant still, but I’ve read advice that on heavily memory limited systems, you should always pick a DE with the most commonly used GUI library (GTK3 currently I think), to avoid having to load two separate libraries most of the time. For example, if your DE is Qt or GTK2, but you have Firefox open, which is GTK3, both GTK3 and GTK2 (in the case of LXDE, or Qt if LXQt) would need to be in memory.