Hey Guys, as you may know, I have three computers in regular use.
1/. I keep XP Pro on a CAD station built for me many years (10?) ago to run 3D CAD SolidWorks 10 - it has never seen the internet so needs no AV and is just so fast! Intel® Core™2 Quad CPU Q8300 @ 2.5GHz 4.0GB RAM. It also runs Serif PagePlus and PhotoPlus.
2/. Have bought Dell Latitude E6420 Intel® Core™ i7-2640M CPU @ 2.8GHz x 4 with 7.7GiB RAM dual boot with ubuntu 16 and Win10. Runs Epiphany and Midori plus DuckDuckGo. GMX free Webmail and Geary mail, Shotwell, Screenshot, Libre Office, FocusWriter, Gimp and Serif PhotoPlus windows software on Linux - the better of the two! FreeCAD etc. Life is good now. Favourite toy at moment is Tails 4.2.2 on a stick packing Tor Browser; so fast and silent that I keep forgetting that I am only in RAM,
3/. Perhaps strangely I find that I am using my rescued (from Vista) HP G60-120EM Intel® Core™ Duo 2.00Ghz 3.00 GB RAM ruining Trisquel GNU-Linux 8. I do not know if this because of the full English keyboard, number pad, scroll bar, larger screen or Trisquel OS. I think I just like Trisquel – even using Ffox again - Abrowser as it is set up so nice out of the box. 
It used to be said that certain software like 3D CAD and CAM could only make use of or address so much defined RAM given also at the time and probably still so, only one OS. Would it then be okay to drop ubuntu on the Dell Latitude E6420 and install a lighter Linux like Trisquel 9 or a medium OS like MX? Would this lead to known issues running FreeCAD or running wine dependent or demanding windows programmes? Would it be theoretically faster or slower?
Could some of the more knowledgeable community members get my head in better shape regarding Hardware vs OS vs Software vs RAM vs Swap, issues – please? Light or Heavy, David vs Goliath, Mini Monte Carlo Rally, Lotus vs Lard, sledgehammer to crack a nut etc. 16.04 LTS vs 20.04, Trisquel vs ubuntu footprint – speed? Is there a sweet spot for harmony? 
Part of design engineering may include FEA – Finite Element Analysis where a component or structure is resolved into a mesh and loading, restraint and material are specified and quantified. The fatigue analysis number crunching was often left to finalize overnight with large or complex items; no problem. This may be reduced to a couple of minutes shortly when quantum computing becomes affordable by engineering firms, but would be a waste of resource just to run CAD, type a letter or edit in the RAW? 
Light or Heavy - Fast or Slow
i think it would be pretty tough for someone without the exact same hardware setup and software requirements to tell you how your system will react given changes like these. one way to see is to multiboot your system and see how it feels. i create 15 to 20 gb partitions for distros i want to test in a full install. i learned that can create some issues with the grub bootloader, but nothing that can’t be dealt with.
that’s a lot of ask in a small space, but without enough detail to help figure out what it is that you really want to know. in general forum posts do better when they focus on a single issue since the discussion can tend to wander as it is. trying to deal with at minimum a half dozen issues will get messy quickly.
for example, since i multiboot i prefer a swap partition. recently ubuntu has switched to creating a swapfile which i think makes sense because it can be grown or shrunk without needing to deal with partition tools and changing uuid’s. but it is hard to tell from your question if that addresses any of your concerns regarding swap 
Okay; @cordx thanks for input on tricky subject. Perhaps wrongly assumed that i7 CPU computers with over 5GiB Ram were made in millions? My used Dell was purchased locally and was supplied as requested dual boot and works fine with no issues. Do not have confidence at present to add more partitions, virtual machine or swap file as tutorials seem pretty heavy duty. Taken a quick look and found GNU GRUB version 2.02~beta2-36ubuntu3.23 and six partitions already so in fear of messing it up. Still, I do have the old HP and Trisquel to use.
Getting so close now as Win10 is just not being used, to format the drive and bleechbit then start again; were it so simple as to have even libreboot with ubuntu, MX, Trisquel and perhaps something minty to evaluate. Guess that gets to the heart of my query; does any member have experience of running demanding programmes on such a computer with a light distro? Are there any inherent advantages or disadvantages in doing so? If Trisquel and MX perform so well why is there a need for a larger footprint?
i think you answered part of your own question/statement with the second sentence. even if the number of computers is as high as you think, you then have to multiply that number by the (i am assuming small) percent that have been prepared as yours has (dual boot). multiply that by the percent of linux users present or (more specifically) active here and then multiply that by the number of linux users of what you call “demanding programmes” like CAD and i don’t see that being a big number personally.
to me this sounds like the often asked question on linuxquestions and reddit “what distro is best for my system” to which the usual answer is “create some live usb’s and see what you like/works for you best.”
one of the reasons i use bodhi linux is because it is designed to be quite lightweight. i have never found anything it couldn’t do that i need (and it does the things that i need well), but then the most demanding programs i use are firefox and gimp.
my understanding about the existence and use of heavier distros and desktop environments is that people simply like them and aren’t concerned about the resource use. bodhi loads at just about 450 mb of ram used and now with a eleven programs open and a ninetabs in firefox it is only using 2.6 gb whereas when i load up regular ubuntu in a virtual machine it starts at 1.1 gb and i don’t personally like or feel the need for that particular distro or de.
Well @cordx may I thank you again for your time and input? Depending on what task you are performing in Gimp it can get pretty demanding as does PhotoPlus. If an operation set for quality rather than speed takes five seconds tops – so what – I can easily live with that or overnight with FEA. Being a fan of small rather than big or gross; it is good to see you and others on it’sFOSS using lightweight flavours – I take your point about numbers and percentages here. All the same it gives me pleasure, or is it hope, to know that there are others out there who don’t need to follow trends, or insist on having the latest whatever, init! 
Now where did I put that Sinclair ZX Spectrum? 
even there i have to admit to being a simple user. i feel a bit overwhelmed by the sheer number of options/seeming complexity of gimp. it is one of those programs that i very much wanted to figure out when i first started using linux full time a couple of years ago because i enjoy taking photos with my phone, but if i can get away with a quick crop and re-save in a simple photo editor i very much default to that
to me one of the amazing and interesting aspects of linux is that there are so many options regarding distributions and desktop environments. i tried trisquel a few weeks back in a vm and liked it. it very much reminded me of ubuntu mate which was my first distro, but with the entirely free software philosophy (if i am remembering that properly) added to the mix.
We use different machines. The pictures you saw here are from my laptop, which is i5 5200U - 8GB RAM and intel graphics, running on Debian Buster. My desktop has i5 8500, 16GB RAM and nVidia GTX-1060, that runs Buster too. There I use mainly Davinci Resolve 
The weakest is an old computer with an E6700 and 4GB RAM. There runs Mint MATE well too.
Browsing the current web is more demanding than MATE or XFCE, so the bottleneck is not the DE. 