The core problem here is not processing power but information: often a central planner just doesn't know what people want or need, and can't realistically get that knowledge out of their brains without using something like a price system. To make this intuitive, consider a farmer with unpleasantly acidic soil. In a centrally planned economy, that farmer had better hope that the central planners know about the local soil conditions and allocated some lime for adjusting the pH. In a less planned economy, it'll be in stock at a local store because farmers in the area buy it frequently.