I think this could lead to subjective opinions, but I'm looking for a general idea on this question.
Is Plasma 5 intended to be lighter than KDE4? Is Qt5 necessarily leading to faster and/or lighter KDE? Is the change related to the possible use on mobile devices and should that mean that a lighter desktop is necessarily the target? I also read at some point that Windows 10 was supposed to be lighter than Windows 7 because it was mobile-oriented: it is not lighter. Is there a similarity with KDE4 - Plasma 5?
When I first started using Linux it was for the purpose of getting back on track a 1 GB RAM laptop, and so I used Xfce and LXDE for that, while keeping away from desktops like Unity and KDE. I was so impressed by the results that for a long time I didn't bother to look at the heavier desktops and stayed with Xfce even on newer machines.
When KDE Plasma 5 appeared I took a look at it and I liked as it looked very good, only to find that a lot of features (like Dolphin add-ons) were mentioned there but not working.
I then tried KDE4 and I was surprised and very much impressed. Finally a really complete desktop and file manager. Compared to that, Plasma 5 now seemed to me an unfinished copy. That will probably change in time (already stable, Plasma 5 is very young, especially in KDE terms), but still, it was even promoted, as I remember, as a faster/lighter desktop than KDE4.
I am talking here about systems that come with these desktops as default, like Kubuntu, Linux Mint KDE and Manjaro KDE.
Anyway, KDE4 (like Linux Mint KDE 17.2) seems to me lighter on my 4 GB RAM computer than Kubuntu or Manjaro running Plasma 5 - and even than Xfce!
Is fast also supposed to mean light? On my machine KDE4 is both lighter and faster than Plasma 5.