buildChunkGraph is slow, but is it really needed for most incremental builds?

[zen0wu]

I've been profiling our webpack build and noticed that buildChunkGraph took at least 50% of the incremental build time (2s out of 4s). My quick read on the function is it's trying to rebuild the chunk dependency graph (from a module dependency graph), but the thing is, most of the times, the module graph (and the chunk graph) is stable since we rarely changes imports etc, compared to the frequency of tuning a CSS style etc. Is it possible to cache this function so that it doesn't take time to run?

Here's a snapshot of the profile

[alexander-akait]

Yes, we already done a lot perf improvement for buildChunkGraph, if you have more ideas - PR welcome.

Anyway we also have two good approaches:

• cache https://webpack.js.org/configuration/cache/ (touch only changed files)
• lazyCompilation https://webpack.js.org/configuration/experiments/#experimentslazycompilation allow to bundle dynamic import by requesting in dev env