Desktop Environment
The desktop environment is the interface of your system. Think of the differences between Windows 7, Vista, 8, 10, 11. That's the Desktop Environment (DE). The good thing on Linux is, that you have large choice about what to use. And you can even install multiple DEs at the same time and switch between them!
All choices are much lighter than Windows. There are differences, but everything will use less than 1GB of RAM, and if your CPU is fit for gaming the CPU usage will very likely not matter.
The most popular options are Gnome and KDE. If you want a good experience, it's probably a good idea to stick to one of these two.
Gnome
The default on Pop!_OS. Polished, easy to use, but customizing is a bit tricky. If you don't want to mess with it too much, it's a solid choice. It supports display unredirection, so you don't even have to mess with composition (but you do loose a little bit of responsiveness).
KDE
Looks much like Windows, and has many nice quality of life features, effects, themes, etc. Easy to customize, but if you're not careful you might break it. Supports disabling composition. If you disable the effects, it's pretty light weight.
Xfce
Light weight and customizable, but not as feature rich and user friendly as KDE. Supports disabling composition.
Cinnamon
People often seem to report problems. Disabling composition might be problematic.
LXQt
Lightweigt DE.
LXDE
Very lightweight, especially low on RAM, but discontinued.
Pantheon
Good looking DE, and only available on ElementaryOS. Does not support disabling composition!