Linux

The problem with older machines is the web browsing, not the system itself. You could use a browser with Java script disabled but a lot of websites will refuse to work.

You have to sacrifice with browser functionality to improve performance.

Puppy Linux is what I shove on old Atom netbooks

Try: https://github.com/marmolak/gray386linux <-- It was designed for really old hardwares.

I’ve already tried MX Linux on an old Thinkpad SL400, and didn’t see any difference from plain Debian.

Because it's the stock Debian + custom themes/skins + some crappy useless minitools. The 99% of packages come from the official Debian repository, the rest are only the rice.

If you have newer machine than a real 386:

• https://kisslinux.org/
• https://github.com/oasislinux/oasis
• https://www.glaucuslinux.org/
• https://www.alpinelinux.org/
• https://voidlinux.org/

Alpine is very lightweight. I think it was built so that it would run well inside docker containers, which means it should be fairly easy for low-end computers to run it.

Afaik, it doesn't come with a DE out of the box, so it won't be very user-friendly

c2d era laptop. first step if you haven't yet, swap the hdd for a low-cost sata ssd if you can. if you have some homeless sodimms, up the ram, too, if it won't cost anything to do it.

if you're going with mx, you want the fluxbox spin; or opt for antix with icewm instead.

otherwise start with a debian base install (no de or extra sw at install), then add only what you need. peppermint is another option--a basic debian with xfce out-of-the-box and little else. it's what i've been using lately on similar hardware.

for something 'different', you could look at slax.

AntiX/MX Linux, I've had great success getting them to boot on systems that were refusing to boot anything else, AntiX is my go-to distro for bringing new life to old hardware, it works with literally anything you throw at it.

I guess it depends on what comes with the distro. If you start off with a basic Linux install and add a DE that is low on system resources, like LXQt, you can breathe life into a machine.

Bodhi, antiX and Linux Lite come to mind.

You can also start with a minimal base, Arch, Debian, Alpine, anything, and then add packages.

Q4OS

antiX should be ok, it's very light

If you want serious optimizations - then Gentoo is your choice. But seriously, there won't be any serious difference between distributions. What really matters here are DEs and browsers. I would recommend some kind of lightweight window manager like i3 or dwm. If you do not want to configure everything yourself, then your choice is lxde/lxqt. Also, you can use distros without systemd (void, artix, devuan, gentoo etc), but that does not matter that much.

Peppermint OS!

Just install Arch without a desktop environment.

So Slackware? If you can cross-compile then maybe gentoo. I'm not sure if Raspberry Pi Desktop is x86.

Bunsen Labs Linux and, for the experience, Tiny Core Linux

I use SpiralLinux on my old Inspiron but it's basically just Debian with some user-friendly tweaks. I guess you could try Tiny Core or Porteus or something really small like that.

alpine and void linux are pretty lightweight.

Bodhi! Another I've found to be lean is Zorin Lite

Puppy/bodhi

I've always had a soft spot for CrunchBang and its spinoffs, BunsenLabs and CrunchBang++.

I used to use WindowMaker on seriously underpowered laptops 10-15 years ago. Seems like it’s still just as efficient. For something more standard interface-wise you could try IceWM.

Another thing to do is build your own kernel without any features you don’t use. Not sure how much of a difference that makes exactly.

I used to use slax, I don't even know if it's still around.

If you have any expectation of privacy, you shouldn't use chromium based browsers. Their purpose is not privacy, and google actively makes sure it will never be.