Having worked around a particular system of Walmart I have to say they are brutally efficient and typically hire good people to run their internal systems. This would be at the corporate level and not store level, can't say much about that myself.

When working with them in measuring the efficiency of the software they were using they were getting significantly higher throughput at significantly lower hardware cost than the average large customer that uses this software. All of it because just a few well learned employees understand how the all systems work together in this workflow and optimize the process.