Maybe because you guys are playing with crossplay disabled and are playing against a lot of bots? The game is more demanding against real people.
I'm running crossplay on. There's the occasional moments where FPS tanks when being shot at by something specific from a vehicle (it is a known bug, happens on PC too apparently). When that does happen, it is usually when I'm getting nuked from a chopper anyway. Otherwise the game runs great on XSX. I'm not running an FPS counter but it feels perfectly fine.
I think people are forgetting that this is running 128 live people on huge maps and vehicles. There's not even anything really to compare it to.