This page is for the Fabric version. If you are looking for the Forge version you can find it here.
This mod reduces the memory usage of Minecraft in a few different ways. A high-level technical description of the changes is available here.
The amount of memory saved will depend on the pack. In version 2.5.9 of All Of Fabric 3 it reduces the RAM usage (heap size) from 1,792 MB to 984 MB.
These values were measured after generating a world with a fixed seed and waiting for two minutes. You may notice that the setup is different from the Forge version, where I compared memory usage at the title screen. This is because Forge populates some caches (which are optimized by FC) before the title screen, while Fabric (and vanilla MC) wait until a world is loaded.
The project picture is a cropped version of this picture.
Should I install this on the server or just on the client?
Some optimizations are client-side only, but there are some rather high-impact optimizations that are relevant on the server too. So you should install it on both sides.
Is this compatible with [other mod]?
Most likely yes. The only exception is Hydrogen, which is compatible with all recent FerriteCore versions on Minecraft versions where Hydrogen is officially released. This means that 1.18+ builds are not compatible with Hydrogen, since it has been officially archived and won't receive any releases on these versions.