Sorry, but this is 2 years of work right here, I don't think I can summarize it unless I make that my day job :D There's plenty of FPS tutorials for Godot on RU-vid, and you can clone this game's Git repository and take the game apart to learn :) - unfa
There's multiple causes, one of them being the fact the game wasn't properly keeping track of used resources, which let to them being freed and reloaded from disk constantly. It is better now, and will be better still, when proper optimization will be done on it. - unfa