Well, in my experience, GTK3 versions of applications such as Thunar and Xfce Terminal feel slower.

You're on my ignore list but unfortunately your text is quoted by others so I see again the illogical statements you make. I'll comment on them briefly.

The concept of 'default desktop' has been discussed in this thread. Note that in the 'xwmconfig' script the KDE session is explicitly configured as the default choice if KDE is installed and you did not define a default session yourself (as root) earlier. So yes, Patrick made a conscious choice here.
During the installation of Slackware you make a one-time selection about what will become your default session, and if it is XFCE then that will also be the default session choice in xwmconfig later on.

About Plasma too complex to maintain by one person, where did you get that from?

It costs time to compile a set of packages but it is not "complex", in fact it is quite trivial to build. A program like gcc, or clang, or firefox, or chrome, or libreoffice, that is complex to compile because their developers change the rules all the time. The KDE developers are writing very clean code and adhere to very strict standards. The number of KDE source tarballs is the only measure of "complexity" but I regard this more as "needs more time than a single tarball".

I stopped providing Plasma5 updates, not because it is "too complex to maintain for me", I think that is an insult to my capabilities. I stopped because I realized that as long as I keep provide Plasma5 updates, Patrick will have no incentive to make his next move to incorporate Plasma5 into Slackware. I keep putting time into these packages without any reward, except that I want to run the latest Plasma5 desktop myself.
The reward will be that Plasma5 becomes part of Slackware so I can focus my spare un-paid time on other worthwhile things such as Slackware Live or making music.

It's the job of the distro maintainer to set the defaults and it is the strength of Slackware that it is easy for you to select something else than those defaults.

If Xfce didn't support KDE and Gnome it would be wimp-y as in Weakly Interactive Massive Particle.

I used to think Xfce was pretty OK, at least with KDE libraries auto loaded at startup, but given it has gone way Gnome, and Slackware dropped Gnome over a decade ago, I'll just stick with Fluxbox and KDE. I tried to go back to Enlightenment (it was my first experience of Linux way back when I first installed emx runtimes on OS/2) but it has juust been too long. I'm too comfortable on Plasma now. I even tried Trinity and though it is snappy, simple and logical, it just lacks way too much that I've come to depend on like Krunner, Kwrite and Dolphin.

If that's the case, then there are some serious issues in the development of the distribution. It's really a pity to hear it and I certainly hope things will get better soon.

I think XFCE is the better choice in certain circumstances. Mostly the decision will be based on personal preferences about visual style and workflow automation.
But computer resources should not be a reason to pick XFCE over KDE Plasma5 - this article is but one example showing that a KDE Plasma5 desktop is comparable to XFCE in resource usage (CPU and RAM) nowadays.
For me personally, the tools and automation present in a Plasma5 desktop environment enables me do my work a lot more efficiently.

XDM is absolutely fine with Sebastien's slackbuild, and indeed is preferable to other Display Managers.

Xfce with kwin managing window decorations and window placement is the perfect desktop for me. But it is absurd to suggest setting a Slackware default, since everybody has different preferences and that's more or less the way Slackware rolls -- leaving it up to the user.

It is unfortunate that the small Xfce team have had to go along with what Gnome and Red Hat took it upon themselves to do with Gtk and the freedesktop but what else can they do?

Please see messages #11 and #17 (paragraph two) in this thread.

Correct, of course, :) but it has been my experience that whatever "root" chooses as the DE, becomes the default for all users unless each user runs xwmconfig and makes a different selection. For example,
if root chooses Xfce, then when the user runs xwmconfig there will be a blue bar running the width of the box highlighting Xfce. If the users wants to use, say, kde4, then they can use an up arrow key to move the blue bar to KDE and hit the enter key. At that point kde becomses their "default," but as previously described, each time they run xwmconfig xfce will still be the highlighted selection.
:)

Mate is the best.

By default I rather meant Xfce but not so simplistic as it is now. It is like offering Plasma with mc as replacement for dolphin. One needs some number of GTK applications to have the same look and feel. Choosing particular theme for Xfce can make qt apps looks terrible. Say there are only okular and xpdf as document viewers.

Little out of subject but I am curious how many people would sign this?

I did. They seem to support my position that there is no default desktop. If you choose to not install KDE, it is not offered in the list of choices. The same goes for XFCE, Windowmaker, Fluxbox, twm, etc.

The closest we get is that the blue bar is on KDE if your installation includes it... Long bow if you ask me.
Yes, but that's not what we're talking about here.

It's a worrying trend.

There is a communication issue there but Slackware Current isn't rolling.