maybe terrains are cached differently than models. you could try to use ent_purge on entity removal, or if no success ent_clone, but not sure, just ideas.
you could also play with limited entity creation per frame when you create a new area with a lot of entities to distribute loading time over several frames, but it requires to do it much earlier than at reaching its border. the best to do it all beyond camera.clip_far. in some game engines far areas are streamed in a background thread.
to be honest, I don't really need such a system currently as I'm focusing on different game genres not requiring such a tricky thing, and I could not write and maintain a MMO game, or a great story based single player RPG that could utilize such a feature.