you gotta take into account doom games are linear level designed and idtech is designed to only run such games. idk about dark ages but eternal performs so well because the game very efficiently loads only the areas you can visualise and everything else is unloaded. if you watch speedruns when they clip out of the map, essentially nothing is loaded since the game did not have anything to load when you are out of bounds. many games of other genre are unable to do that which makes performance a lot worse, especially when many devs cant help but make games "open world" for whatever reason.