Sounds like a PCI express limitation...
Not a hardware limitation. Just the way the game loads and culls assets. If I increase draw distance, it happens in a wider and wider radius around my camera. Thinking of it in terms of a "square grid", if I'm loading a 3x3 zone, and I want to increase the draw distance, the engine will have to up it to a 5x5 zone next, then to a 7x7, etc.
But look at what's happening to the
area: 9 --> 25 --> 49 --> 121... So, if we're to say there's an average of 5 NPCs per "grid zone", then the engine only needs to process 45 NPCs total. If we push it out just two more steps, the engine has to process 245 NPCs -- simultaneously. I've exponentially increased the load, which is taking more and more resources up on the machine. I can expect an exponential decrease in performance until the engine finally crashes.
Thus, to create the illusion of a continuous world, I decrease the range at which world assets are drawn or culled, and that, in turn, ensures the engine never hits "critical mass" with NPCs and stuff. Now, the engine can stream in assets more fluidly, but the distance at which assets are drawn / culled decreases, and I may notice some fade-in or pop-in.