This is my rule of thumb and process to choose DE and distro:

1. Find the CPU model and do a google search with it and the word passmark. The passmark page will tell you how fast the cpu is. If it's between 500 and 1000, use XFce as your desktop environment. If it's between 1000 and 2500, you can use Cinnamon (Linux Mint). If it's more, you can use kde/gnome. If it's less than 500, use LXQT or LXDE.
2. How much RAM there is in there. These days, you need a minimum of 4GB of browse the internet (the DEs/distros themselves might use less than 1 GB of RAM, but the moment you open a web browser in this day and age, all hell breaks loose with memory usage). For best performance, 8+ GB is better.
3. Ensure that it has over 16 GB of a drive. At 16 GB (as in some old Chromebooks), only Debian fits these days (with 6 GB free space after installation). Mint and the others prefer over 24 GB (both fedora and all the ubuntu-based ones are too big to fit in 16gb without issues -- debian fits).

Using these rules, I've converted many laptops and computers for my family here in Greece, installing the most appropriate each time. The least powerful computer was my mom's old laptop, with 16 GB internal, 2 GB of RAM, 600 passmark points. As long as she's only opening 1 tab on Chrome (Debian/XFce), she fits in the 2 GB RAM without swapping (most of the time). I use Chrome and not Firefox for these older laptops because Chrome uses LESS memory than Firefox (there's an additional setting for it in the settings to help the matters more), and its youtube playback speed is much better too. I use firefox on more powerful computers, and it's my default too, just not for underpowered computers.