It won't always be faster. In fact, I'd argue if you had to lay out the whole thing by hand you would almost certainly do a worse job than "automatic" place&ro

It won't always be faster. In fact, I'd argue if you had to lay out the whole thing by hand you would almost certainly do a worse job than "automatic" place&route. It's a matter of distance between components on the die, and transistor sizing. At the scale of complex integrated circuits, it's just about guaranteed that an automatic route will do a good job, but stands a good chance of messing a few hotspots up. I'm not an expert on the algorithms used for automatic routing, but there is some random guessing involved. You can even provide a seed for the random number generator. More often than not, a hybrid approach is used where the designer gives broad recommendations on where to place components and lets the autorouter go from there. Then at the end, the designer may go in and tweak some hot spots, or scrap it and start over with a new seed.