This has been a habit of mine for many years; installing various distros on old hardware. What is meant by "lightweight" can be "slippery" and often refers to two things. One is low CPU/RAM usage and the other is low disk space. How I think about it is will it prevent the machine from being a doorstop?
Or, how well will it run old hardware? What exactly is old hardware changes too; think of Moore's Law, but in a more general progress of technology sense. To cover that, and make this pet project of mine more useful, below are Distros against test machines I have lying around. Maybe it is weird, and it is definitely subjective; please check out the comments below.
Table of Contents
In the end, its all pretty much "impressions". If anyone tries to imply its more then opinion, well.... I want to be nice. :-) It is hard to balance how much of a modern tool it is vs. does it provide enough tools and stay light weight. I want to give credit when the distro does what the creators want it to do as well. For that reason, I use the default GUI/DE and the default apps initially. I do try other WMs/DEs (OpenBox, LXQT) and the apps I prefer when available.
Anyways, here is a list of things I run through:
Installation process - is it easy / intuitive?
Setting up GUI / environment - can I get things the way I like it?
Browsing App(s) - how well do typical websites load, can I do online email, can I do online document editing, what's up with streaming, etc.?
Media Playback - how well does local music and movies get played?
Media Editing - is it reasonable to edit graphics, music and video?
Office Suite - do the expected apps work for typical docs, spreadsheets and presentations?
Combine it all, stir, pick a number, type it in above. If I have time, my blog may have more detailed reviews of the distros individually.