I would personally point to a framework like Meteor as my example. Having a framework handle all of the reactive parts of your code with no additional work, and giving the end user a way to simulate database updates on the client side while it's being processed server side is a huge win for performance.

Now of course none of this was impossible to do before, but the advent of all major browsers now supporting web sockets, and the existence of this framework and community which has already solved all of the hard parts of the implementation makes it actually feasible.