The reason central planning fails (in societies or in companies) is because the people doing the work at the "bottom" have a lot more knowledge about how things really work than the people making the decisions at the top. A lot of that knowledge isn't even easy to capture - it's tied up in skills, techniques, oral history and obscure trade secrets.

In other words: the problem isn't likely to be speed/power of the planning computers, but how you get the information that needs to be fed into them.

I think this could also explain why managers who worked their way up from the bottom tend to do well, whereas professional executives who are helicoptered in to companies fail.