Most efficient is the DE/WM I use, obviously. 
Currently it is i3wm; xfce or gnome a few years back.
It really doesn’t matter - almost all is customisible to the extend I need it to be and when I learn to use it then it becomes “the most efficient”.
It is the same situation like a keyboard layout. I always wanted to learn more efficient keyboard layout than qwertz but after so many years it is still the most efficient for me.
Definitely Windows 11 
The most “efficient” DE is the one that fits you and your workflow. I navigate with my keyboard 90% of the time so GNOME is my favorite.
For me it is Gnome, I like its simplicity. I don’t do theming other than switching to dark mode and use very few extensions. I also have two touch screen laptops and Gnome with onscreen keyboard works great on them. I like the workspace design in Gnome, when i login for the day i just open every app i plan to use each on their own workspace and just leave them open and alt+scroll wheel to each workspace as needed.
Plasma just gets out of the way and lets me do the work I expect to do.
What I still hate, and this is an across-the-board thing, is the QT vs GTK crap. Neither get it 100% usable, neither have a monopoly on the major applications, although more seem tuned to GTK, and God forbid trying to get consistent theming. Why can’t these apps have a flexible choice of graphics library, so Chromium looks the same in QT as GTK for instance? And then there’s the file chooser dialog boxes, which are different according to the phase of Pluto (save files received in Viber, then save files received in Signal, then save files downloaded in Vivaldi, then save files created in Kate. Every single one has different dialog box, with different saved folders/bookmarks. Is that really necessary?)
Yeh, I know, if i wanted complete consistency I should go to MacOS, but yuk. I like my leenuks, and so wish for these things to be sorted out somehow one day.
Just looked down through the comments.
Interesting trip down memory lane.
With relation to the initial topic - I don’t think an efficient system exist - unless you build it ground up the Arch way.
If you choose any distribution you are presented with an initial layout defined by the philosophy driving the developers.
EndeavourOS is one the best Arch derivatives for this purpose.
I have been touching down from time to time on Linux for decades but due to my professional life concentrated around clients with Windows environments - a few with macOS mixed in - and therefore it has been difficult to get a firm grip on Linux.
All my touch downs with Linux in the past was very often accompanied by graphic driver issues. My atttempts to learn was often worked against due to lack of resources that could be dedicated to Linux.
The incoherent appearance of the confusing mix of gtk and early qt was also turnoff.
The early attempts on using linux requiring extensive knowledge of compiling software from source made me turn to OS/2 with were better but with no tools.
So I found myself going back to Windows again and again. My earliest experiences was Clark Connect Firewall/Gateway/Proxy and Suse Linux Enterprise edition with SLOX. But again the pain of configuring the hardware - the slow internet connection - my lack of knowledge in C - all stalled my progress.
It was only when I learned about Arch around 2011 and learned how to put the Linux puzzle together - how the filesystem holds different pieces - the filesystem concept of everything is a path even devices - where the local system wide configs are stored - how the configs are prioritized from /usr/ → /etc → ~/.config and ~/.local - that is were it began to make sense.
When the great picture dawning I began looking for an easier way to install arch and my first experience with an Arch derivative was Cinnarch.
I really liked Antergos Gnome edition and stayed with Gnome for a long time. Some of you may remember that Arch has not always been as stable as it is now and back in the early Antergos days - Antergos was not that stable either.
I discovered Manjaro back in 2015 and oddly enough given the very early history of Manjaro (which I have not experienced first hand) my Linux experience stabilized - I still don’t know why - but it did.
Despite my Python knowledge was null and void - I volunteered when a team member asked for help in rewriting the pacman-mirrors code - so I started learning Python - I am by no means an expert - and I willing to admit I rely a lot on research when I code - the topic is always covered - perhaps not in details but generic examples on how to solve a challenge is often invaluable in the learning process.
The April fools day - where the team created the prank to drop x86 and concentrate on ARM - I got invited to join the team - I hesitated - I didn’t want to be part of prank - and despite my coding skills in deprecated languares I had no desire to be part of a distribution - but I was flattered that a no-one - a blank sheet - like me would be considered so - the flattery worked - I got a manjaro email address.
That was a steep learning experience - while I could draw on the core team if I had questions - asking for help is not my strong side - I am a stubborn idiot who takes pride in learning by reading and doing so I decided to figure out everything on my own.
So I did figure out how to build the settings packages - how to follow the tracks down through PKGBUILD after PKGBUILD to locate the source - then learn to modify and created new PKGBUILD and then use those to build an ISO and that was done without any documentation - other than reading a collection of undocumented bash scripts - learning how ENVIRONMENT variables work when running scripts - so my know very deep knowledge of inner workings of an Arch based distribution - I have accumulated through the experience of being a member of the Manjaro Team.
This resulted in the resurrection of the Manjaro Openbox ISO and builing that has been very, very educational.
Not so long ago I retired from building packages and ISO - I still maintain the ISO-profile source and pacman-mirrors - but the ISO are now built using Github runners - and it has been a relief not having to worry about the bugs getting packaged and deployed to public 
.
So an efficient ISO is impossible to build as it will only match the select few who shares your opinion on efficiency.
This is the force of Arch - and Arch or an Arch based distribution without to much bias to create their own exeperience - you create your own efficiency and control your workflow down to the sligthest detail.
This is also why complete beginners should never - unless ready to do some university style study and dedicat themselves to the process - start with Arch or an Arch derivative.
I guess that would require me to efficiently use my DE, which I probably don’t. I don’t use any ‘power user’ features (keyboard shortcuts, hot corners, etc).
Started my Linux life with Mint/Xfce, which I found clunky and unlogical. Then was told I’m stupid and I must use Manjaro/Mate. Heaven! Since then I’m on various sorts of Archy distros. Right now EOS/lappy and Manjo/desktop but always stayed with Mate.
Why?
Coz Mate is super boring and gets out of my way and isn’t an attention whore. So I can get stuff done.
This a hundred times!
For what it’s worth, all I really want for a graphical environment is a simple window manager, and for that, the IceWM is sufficient. I use Xfce as a desktop because it is somewhat scalable, and I can use it “on the lean end” of the scale.
As far as desktop functionality, I appreciate the flexibility of KDE. In the early days, Xfce and KDE arrived 1-2 years before the GNOME desktop was usable; Xfce was eventually rewritten to use Gtk+, so in that regard it shares a few libraries with GNOME, but is otherwise much leaner, at least in basic form.
I got away from both KDE and GNOME when each of the projects had major rewrites and years of questionable stability; that’s a long time ago for both, but I never really got attracted back because I don’t need the features. Given the choice between KDE and GNOME though, to this day I’d still take KDE Plasma in any of it’s variations in preference to anything on the GNOME tree, except tools that only utilize the basics in the Gtk+ libraries, and the reason is that I really don’t need even that much for my simple use cases. I wouldn’t even use X if it weren’t for Web browsers, and I’m comfortable with command lines because my UNIX and Linux background predates graphical user environments.
Funny - that’s why I use XFCE! To each their own…
After Mate and Cinnamon my 3rd fave desktop. But it can’t do many things Mate can do and is comparatively primitive and cumbersome to operate.
Than you @root What an amazing response. So much I can relate to, even my brief dalliance with OS/2 Warp, and came to the same conclusions about its limitations.
That was my thinking when writing the topic.
Precisely why I chose EOS over all the Arch derivatives.
I never saw Antergos apart from screen grabs.
Even though KDE Plasma seems to have endless granular control over configuration, Gnome has still let me do almost everything I need and stay efficient.
you slew some big giants in your memory lane. I would be satisfied with writing my own PKGBUILD and it’s been on the to do list since meeting the amazing
I keep finding that out the hard way 
It’s not that I want to be a power user. At my age I am not sure there are enough years left in me, but certainly, we all want to be more efficient in whatever we do so we have time for other things in the time we have left. That’s why i specifically said efficient not most customized. I am trying to master how to bend Gnome to my will, but some of their their documentation is so cryptic and outdated, and I think the developers want us to use it as it is.
Cheers!
Nice looking cats…
KDE all the way for me, even on the wheezy i3 4Gb laptop it works well.
Of course it has always been KDE on antergos/enos/garuda and that may be a factor.
I do disable baloo on all my boxen
Fortunately not mine. 
Those cats ARE nice!
It doesn’t matter what environment, desktop, window manager or command language interface is used.
I started with ed and UNIX.
I also have used TECO, mainframe systems, card readers with punched cards, paper tape readers with the Dartmouth Timesharing System, DTSS, MULTICS with Emacs, Slackware with 14 images, which I was fortunate to fit on bootstrap loader on one 3 1/2" disk and the others on the other disk instead of 14 separate “floppy disk” images.
So Linux anything is fine and EndeavourOS is straightforward and no problem at all.
I like the little orange one but they are both nice. 
At different times we’ve had a cat similar to the one on the right and later a ‘sweet Boy’ like the one on the left.
It’s my aging Mother who had the one on the left ; she called him Stewart, my sweet boy.
I inherited an orange cat named Fluffy and we had an orange cat a long time ago named Meesha.