Obviously, cheaper workers work for less, but that's not exactly efficiency, (1) more a question of fairness, (2) can be done domestically (low-cost immigrant workers) and (3) there's plenty of cheap workers worldwide, not just in China.
Another potential advantage could be, factories are closer to natural resources, but it seems to me that that's not as much a question of efficiency, as it is of lax (enforcement of) environmental standards. Which I personally don't see as a net gain, even if it results in cheaper production.
The only real advantage I can see is concentration of skill - e.g. people say that you basically have to develop electronics in Shenzhen, there's simply not enough talent in the US and EU - but I wonder if the efficiency gains are offset by innovation losses. Again, IMO it would be better to have a local high-skilled population of engineers & entrepreneurs.