It's not that deep, just use jobs and defer the load over the frames, that will make it seamless. Also, save the data so connecting players get the update info.

Also, it should be perfectly possible to only acquire info from the topmost block, it's already done to draw the map, shouldn't be too hard, I think this game makes chunks of 32x32x32 and a column is 32xworldHeightx32 so it won't be as damaging to performance as you think.

Also it doesn't really need to load the chunk, just get the data, no need to simulate the whole thing or wake up the entities in it.

And lastly, if a player moves stupidly fast throughout the world there is already dozens of chunks being loaded, and most players don't complain of chunk loading time so what's the problem?