Level streaming detailed explanation

Id like to ask for some clarification on how level streaming works, as iv encountered some interesting things which don’t fit with my current understanding of them. Basically i have a relatively small but high-detail environment in unreal consisting of a building and its interior (Its a VR arch-viz project essentially). I’m using a bunch of level streaming volumes to help performance by not loading in things i cant see (like the other side of the building, rooms on floors the player isn’t on, etc). Iv got this all set up correctly, as i want it, and it works in the sense that the number of actors loaded in at any time is reduced to the expected number. However it wasn’t giving me close to the performance boost i was hoping for, so my next move was to make LODs for everything, and that helped drastically.

But the thing is my environment is so closely packed that the actors in the currently loaded levels mostly dont get a chance to use a low-poly LOD, so the performance boost must be coming from the LODs being used in the unloaded levels somehow?

From my understanding of level streaming the other levels shouldn’t be loaded up at all, for LODs or otherwise to impact performance, so im wondering if theres something im missing about how level streaming works, or is there any indication in what iv said that iv done something wrong?