Backport initial page load performance improvements to JL 1.2.x

[reesscot]

Problem

• I'm trying to make JupyterLab 1.2 load faster on initial page load and I've seen that a number of improvements have been made in JupyterLab 3.0 to split the vendors~main.js (JL 1.2) file into multiple bundles. In particular I noticed that there is now a jlab_core bundle. I tried backporting this to 1.2 and it reduced the size of vendors~main.js by about 1.5 MB.

JL 3.0 webpack cacheGroups

optimization: {
      splitChunks: {
        chunks: 'all',
        cacheGroups: {
          jlab_core: {
            test: /[\\/]node_modules[\\/]@(jupyterlab|lumino)[\\/]/,
            name: 'jlab_core'
          }
        }
      }
    },

Proposed Solution

• I would like to submit a PR to backport bundle splitting optimizations from JL 3.0. However, are there other Webpack changes in 3.0 besides the jlab_core bundle that I missed? I also noticed that 3.0 is splitting the node_modules dependencies into separate bundles as well.

Additional context

JL 3.0 assets optimization (DevTools Performance)

JL 1.2 with jlab_core cache group backported from 3.0. (DevTools Performance)