Since you can have a local cache of objects of interest to the user, many subsequent user interactions don't require an RPC. For example, you loaded this month in the calendar, but once loaded did an extra RPC to fetch month N-1 and N+1's content. Now when the user visits month N+1, you can just render the calendar with that content without any requests or apparent delay to the user. Further, if the user clicked on a calendar day, you could show the day details with no additional RPC.

I quote 10ms as client side code to produce the new HTML and browser painting events can take time, but if you used a canvas or similar you could cut down even further on that render time.

No, the 10ms is because you don't have to hit the server on every page load.

But presumably you'd need to hit the server for new content. The RTT on that alone would take more than 10 ms.