I wondered if the level streaming loads directly from the HDD. My first thought was if that would not be to slow, or if the chunks are just loaded into RAM and asynchronously processed if appropriate or something I didn't thought of yet.
A lot of engines uses both in a two steps loading approach:
Load groups of compressed chunks (level/world sections) off the storage device, slightly in advance if possible.
Decompress the small chunks within from RAM into an active usable format when they are needed.
Keeping the larger compressed chunk groups cached in RAM for a little while in case the player walks back and forth.
In your larger groups you can put list of local assets (textures, meshes to instantiate, etc) to preload as well.