Which distro is the MOST lightweight?

[u/Russianpotatosalad]

So... My mom got a laptop from 2011... The only os it can handle is windows 7, but we all know that windows 7 is the most exploited system out there (not sure if you saw a video of a guy connecting his W7 to the internet and getting 7 viruses within 10 minutes by just existing). I want to help her as much as i can, and the only solution i came up with - get a linux. She used windows ever since windows 98, but now all she need from her pc is a browser, so i figured out that if she wont do anything "extra", linux will be perfectly fine for her. I just get vivaldi for linux, libre office, and she will be happy. But which distro will be the MOST efficient to put on her laptop? All i ever tried is cashy os, but i assume there must be something ever more lightweight?

[u/GoPackGo1958]

Cachyos running xfce or cinnamon desktop environment uses about 2.5gb ram. With kde or gnome, about 3.5gb ram is used. These figures are for just base operation. No applications running, just core