Batch sparging doesn't require a slow drain, even thought that may work for you. I batch sparge in a round cooler with a manifold made of copper pipes that doesn't really clog unless I have a ton of wheat in a recipe. So I don't drain slow. I vorlauf about three quarts, put it back into the mash, then let it rip. I then sparge and let it sit for a few minutes, vorlauf again and drain. I routinely get 75%-80% efficiency. Everyone's system is a little different, so do what works for your system. But for me, time is one thing to consider. It already takes a long time to do a batch, I like to figure out how to make it shorter by cutting out stuff I don't need to do. I get through the first runnings and sparge inside of 20 minutes, then onto the boil.