Justin I think it uses less RAM. It's using just 0.71 GB now with just Firefox and the system monitor open.
I also have a old 2GB graphics card with slower memory.
I think whoever makes KDE have made it lighter as the project progressed. Don't know how.
Solus Plasma is just as snappy as Solus MATE, on my system, that doesn't really makes sense to me, as Plasma has more pretty transparent stuff.