I do accept your reasoning, which is why I wanted to see some reliable usage numbers. Given XFCE's performance in the (non-definitive) popcorn results above and in this LinuxQuestion's 2015 survey (http://www.cio.com/article/2881172/survey-says-kde-plasma-is-the-most-popular-desktop-linux-environment.html),
I think it's reasonable to consider XFCE as a contender even if it is borrowing GTK+3 from Gnome. Until Gnome / KDE provide a lightweight version of their DE, the demand for a 3rd lightweight DE will always exist. I'm not advocating for it - just giving you all options.
I think we all agree that the top priority is to be able to make this transition with as little disruption to Whonix's core objectives as possible. Which makes me reconsider one of the original requirements of having some consistency with Qubes. I'm not sure I see a need for that at all. GUI apps should not be accessible from dom0 (and won't be after gui-domain is implemented). The only program I've ever run in a serviceVM is gnome-terminal and NetworkManager (which is pretty consistent everywhere).
So why do we need app-consistency with appVMs that many Whonix users will never use anyway? Is the motivation for this so that non-Whonix Qubes users can make the transition to using Whonix? Whonix is a distinct environment with a distinct purpose. If anything, having a separate DE reinforces that new users need to re-evaluate and reset their old computing habits.
I tried it also so I could compare apples to apples on my test system.
install kde-standard virtualbox-guest-x11: laggy animations as you said. on heaviness-scale, similar to gnome-flashback. not as heavy as gnome-shell, not as light as xfce or lxqt.
changed compositor from
XRender. whoa! (kde) fps went from 15 to 60. Strange because OpenGL causes no slowdown on KDE4 in Virtualbox. UI is very responsive - feels just a tad slower than XFCE / KDE4. I've got KDE4 & KDE5 running side by side and apps (torbrowser, dolphin, konsole) take perceptibly longer to launch - but not so long that it's disruptive to workflow. For example, torbrowser takes an extra 1-2 seconds on my system. Initial loading of the desktop is very slow.
I'll have to go back and read the qubes' KDE5 complaint thread. Maybe they weren't happy with interaction with qubes-gui? Personally, I like KDE-apps and KWin and performance is fine AFAICT. I thought installing GTK applications like LibreOffice would pull in a bunch of Gnome dependencies anyway but my previous post seems to dispel the notion that a GTK DE would be much more efficient in terms of number of packages / disk space.
minimal stretch + kde-standard = 4.0 GB
install gnome-terminal nautilus seahorse gedit
110 newly installed, 267 MB of additional disk space
157 newly installed, 613 MB of additional disk space
install libreoffice + 4 gnomish-apps
262 newly installed, 879 MB of additional disk space