Framework: Fast Builds

[apeatling]

One of the original goals of Calypso was a fast developer environment. For this to continue to hold true, the builds need to be fast, ideally under a second.

We should parallelize all we can, turn on HardSourceWebpackPlugin by default, profile what's taking the time, and also seek external help from the webpack team.

This might also be an area where we could contribute to the project.

[apeatling]

Keep an eye on Webpack 5, this will definitely help build speed without us doing anything: webpack/webpack#6527