Disable "streaming textures" in viewport

So far, all I have in my scene is a landscape, a player start, and some lights. After my most recent lighting rebuild, every time I move the viewport camera a bit, “streaming textures” pops up and stops me dead in my tracks. It’s a really unacceptable slowdown, because it comes up so often that I can’t actually do much of anything. Is there any way to disable this? What does it even do, anyway?