It’s not one big optimization, it’s a product of Id actually having some of the best UE developers on the planet being able to tweak the engine to run like a beast. Each level is crafted from the ground up to allow for some sweeping optimizations revolving around actor loading and culling, and the game uses proper light baking to allow raytracing to handle marginal calculations instead of explicit path tracing every shadow. It’s a lot of little things that all take impressive amounts of skill and management to pull off effectively, a lot of this stuff is implemented poorly in other games and it shows.