Dynamic population is computed at render time, right before sarting rendering, acutally, and if you use an infinite terrain, it will of course take quite a while to populate. But this will always be the same amount of time, whether you are in preview, or ultra, it's like "preparing" radiosity. Maybe e-on should add an info text at the bottom left of the interface saying what's going on, instead of just "preparing".
Now about your display, maybe you could move the instant draw slider a bit more to the right and see if the terrain previews faster. But I agree, these inf terrains are slow to redraw.