One thing I've noticed, at least it feels so, is you're likely to get much faster spawn of the heavier NPCs if you're not close to the RES marker and inside the asteroids; if you float just above and away from them, perhaps because there's more room or less Police about, they seem to come one after another... you need to be in an Anaconda spawning instance all the same however. I made 5m in just over an hour yesterday as it was just Python/Clipper/Dropship/Anaconda over and over again, but I did reset it 2-3 times to get that.