Millions of people who plays MMO or multiplayer games still experience lag and latency currently. What makes you think that wouldnt happend with cloud computing. We are talking about up to a million people logging on and playing a game that uses cloud computing. This is not the same thing as streaming movies through Netflix or streaming games through Playstation Now.
When you talk about real life scenarios and real life use of cloud computing, you have to think about the worst case scenario. This worst case scenario is that it can be possible that every single XB1 owner can potentially log on the server all at the same time and use the cloud computing. Microsoft claim to have 300,000 servers (possibly more) this is not even close to enough servers. Remember Microsoft is a software company, not only they will be using this for XB1 but they will be using their servers for other application as well.