SSDs are what breathe new life into old PCs. And they are quite cheap. My wife's laptop turned from extremely annoying to decent, almost pleasant to use.
Think that's a different class of old PC. They're talking about computers with 512MB of RAM, you're getting up towards 4GB before spending money on an SSD is going to be a sensible I'd say.
OA uses a Thinkpad X40 as the target machine. A 1.8 inch Hitachi drive with PATA interface and with 256 or 512Mb of RAM soldered to board with a socket for one further RAM stick. This is a 12 year old design. Probably best used with what it has rather than spending any money upgrading. A good test target for a 'light' OS.
A Thinkpad X60 or later (dual core) with 2Gb RAM can run a 'full fat' Linux (e.g. Ubuntu or CentOS/Fedora) OK if not slick in my experience (and yes I did once compile a kernel from source on an X61s - took a few hours). An SSD makes a difference in that kind of machine.
Your comment about the kenrel taking a few hours to compile made me chuckle. I went through a Gentoo phase. Nothing like compiling your compiler to compile your compiler to compile your system. Took days.
And before that I had a machine with 24 MB RAM and a terrible habit of recompiling the kernel or running BSD with ports.
at least yours finished. Mine would run just long enough to waste most of my day, only to error out because one lib wanted python-3.2.785.2.r58 but could only find python-3.2.1435.214.r3
I was using Slackware around the time that distros were switching from xfree86 to x.org. Not knowing any better, I was trying to run it as a bleeding edge system. I think I took a week or two manually upgrading each library one at a time to make the jump to the new X server.
Apparently I didn't learn my lesson; the next distro I used was Gentoo.
An SSD and stock Gnome 3 Debian in my X61s (2Gb RAM) with its Atheros wifi card goes very well, so I'm hardly surprised that your machines work well with a less demanding desktop environment and it is good that you find them of use.
Care to share the output of pstree -l -A from a fresh session? That would tell us what processes are running by default (cups&c).
I've found that an SSD is significantly better for most "store bought" computers/laptops.
They advertise "gigahertz", RAM, drive space, and sometimes screen size.
So everything else is left to the cheapest possible part. Updating the 5400 RPM HDD to an SSD has completely changed every single laptop a family member has brought me as "too slow to use".
I've also found it's damn near impossible to buy a cheap laptop with an SSD.
This should definitely be investigated before investing time getting an alternative OS up and running, along with the troubleshooting and training that comes with it.
Upgrade to the latest CPU for the platform. For $25 you could gain an extra 1+ Ghz or more cores.
Max out ram, for DDR2 systems this is usually 4GB - 8GB. 2GB sticks can easily be had for under $10 a stick.
Replace hard drive with SSD. Especially if it's a low RPM hard drive. 128GB SSDs can be had for $35 on sale.
The above upgrades will easily boost performance whatever OS is already running on the hardware, and total time to install should be 1 - 2 hours.
There are a lot of very respectable laptops available (off-lease and such) for relatively cheap.
I picked up a ThinkPad T420 a few months ago on eBay for, IIRC, $180. For $250 all in, I had a nice i5 laptop w/ 16 GB of RAM and a 120 GB SSD. I didn't need it and don't really use it for anything but it's great when I need a spare or somebody else wants to use it. It could easily serve as a great primary machine for an average user (i.e. e-mail and Facebook / web browsing), though.
Unless the system is too old to contain new drivers, or you're switching from x86 to arm, you shouldn't have a problem. I've done this multiple times, including a gradual upgrade from Ubuntu 8.04 to 16.04.
I am actually more worried about security with modern Intel processors than with old ones.
Intel Management Engine makes me very uncomfortable in trusting my machine.