freecicero: AIUI, KDE has more packages than XFCE which allows KDE to be the "blingier" DE with more eye-candy than XFCE. As these packages all involve software that needs to run if it is to be of any use, I also expect this will result in more processes running. IMHO, this is why KDE feels more sluggish than XFCE with similar hardware.
I use similar logic with Operating Systems in that the older systems were far simpler than the newer ones, with correspondingly smaller file sizes. I have no scientific justification for this but my experience over the years has taught me that OSs just get bigger and bigger. Inevitably, these will also require more running processes than earlier OSs and so I believe this is why they require more powerful hardware (both CPU and RAM) to run smoothly.
As the changes between releases are incremental, there probably isn't that much of a difference in hardware requirements between Stretch and Jessie, although the difference between Stretch (9.0) and, say, Woody (3.0) is probably quite significant!
Bootup/shutdown speed is something I never really consider as my machines run 24/7. However, as far as running programs such as LibreOffice or Firefox are concerned my opinion
is that the lighter the system, with the fewest processes running, the better. This will then ensure that your wanted programs will have the maximum resources available. With modern, powerful, hardware, this probably will not make much difference. But with older, more limited, hardware this could be significant.
In summary, my (unqualified!) opinion is that, if the best performance is wanted, older hardware works best with older OSs. The only caveat I would add is that the OS must be still be supported - which is why I still run CrunchBang with its Debian Wheezy (old-stable) base.
As far as Puppy/JWM is concerned, just give it a try and then please report the results of your comparison back here. It would certainly be interesting reading.