Webpack 2 Dynamic Imports are Very Slow

Created by: gaearon

While we officially support dynamic import() proposal, it is reported to have significant build performance issues (https://github.com/facebookincubator/create-react-app/issues/2172#issuecomment-301791999, https://github.com/webpack/webpack/issues/4636).

We are going to de-emphasize their support in the documentation until Webpack fixes them. I will disable the require.ensure deprecation, and will track re-enabling it in this issue.