Do some hand-profiling with React 16.5

[markerikson]

The React DevTools have been updated with the new profiling capabilities, but they require a release of React that implements the interaction tracking backend to make those functional. React 16.5 should be out tomorrow with that included. We should start using that to do some investigation of the different builds. (We'd probably want to change the behavior so they don't automatically keep updating as we're viewing, or maybe we just snapshot a short time period and then inspect the results.)