> So it will be equally fast if only a few people use the service (e. g. outside of populous areas), and much slower otherwise.

Since Google isn't actually deploying the cell networks and they actually use a mixed set of providers, I am not sure if this assumption makes sense.

If the mobile network T-Mobile is overloaded with T-Mobile customers and has a small fraction of Google ones, then Google will tell T-Mobile to throttle the customers that exceeded their contract limit?