I agree, it has state of the arts systems, however, you should remember that the game was released many many years ago (in 2017), most of these other systems you mentioned are already there since day one, it only recieved Lumen, Nanite and VSMs recently.
The game has simple animations, basic physics and destruction, average looking particles, the geometry of dynamic objects such as characters, weapons and vehicles is also average. It has no advanced physics or hair simulations. This is why I called it relatively simple compared to something like Last of Us Part 1 or Uncharted .. etc. Fortnite is simple by design as it's a massive multiplayer shooter that needed to be relatively lightweight to accommodate all the crazy action.
Yet despite that, you lose huge chunks of performance whenever the game transitions to the new features, first you lose fps when you enable DX12, then you lose more when Nanite is enabled, then Lumen then VSMs, in the end you end up losing 3 times your fps in the process when moving from max DX11 to max DX12. A 4070Ti went from 217 fps at 1080p to 78fps. Imagine that. A 3090 class GPU doing 78fps at 1080p.
Which is why I am saying that this Immortals game is not behaving out of the ordinary when compared to Fortnite. That's how things behave now with UE5, people shouldn't really be shocked.