Placing a block that blocks all sunlight for a small area is peachy. However, when placing a block in a whole that provides sunlight to a mass of blocks, the sudden decrease in sunlight for all of those blocks makes a noticeable amount of lag. Test this by creating a large sheet of opaque blocks one layer thick and then break and replace the central block. This may seem small to some people, but when making maps with large areas, we can't afford to wait 1 second to place each block.