It could be as a result of optifine, I've never used it, but if it's doing something different that might cause that bit extra strain on loading.
I have it frequently nowadays working around my place - I assume just because it's a lot of building in the area and it takes a while to load the chunks when I move into a new one. Usually they will eventually load properly, you just have to give them time. Sometimes patches still won't load properly and putting torches or other blocks on there can help bring it back or if that fails reconnecting generally fixes it.
Back in the early days (before beta), it pretty much always happened when you moved into a new chunk (if it didn't just crash the game on you instead).... you'd have to wait for the world to render properly around you. It was the only way I ever saw slimes until recently
Now it's kinda handy to see abandoned mines and strongholds heh
But that is a different issue to the missing chunks "world hole" thing, which is where there will be a square "hole" right down through the ground where you can see a cross-section of the land..... and trying to step into it usually makes you bounce around.... which are also a "feature" that has always been around
(though again they used to be much more frequent in early days), and can usually be fixed with either placing a torch at the edge of the "hole" or disconnecting and reconnecting... or just ignoring them, unless they are in your way.