Well... Not so much "load in", as in "finish rendering", I'd presume (except for streaming something recently flushed back from cache); As far as I understand, the terrain generating system composes these patches of ground in the background as you go, with a slider in settings letting you decide how great a portion of GPU cycles to assign to that function, taking away from rendering frames... :7
EDIT: Hmm, I guess I should amend the above. -With the increased dependence on prebaked bitmaps for geological features, the terrain gen is of course going to be held back by having to load all of those, that are needed for a given patch of ground, before it can generate anything, so there is that... :7 ...I wonder whether they are mipmapped...