In a world has a lot of redstones, chunk load and update very slow near the redstone construction which has a lot of redstones. But when start placing block it update instantly
Example: Connect redstone to redstone lamp near a huge redstone construction. When I turn lamp on, it takes a long time to display a lamp turned on, but when placing block it display turned on lamp instantly

And affect lightning too, when I cover, the light doesn't go away instantly, it takes time, placing block will make the light go away instantly

Also loading world at that huge redstone construction is slow, other place load completely