(based on 4.39.2)

Performance

Compilation/NormalModuleFactory.file/context/missingDependencies are now LazySets
Performance improvements in deserialization
Performance improvements in module graph building

Features

add cache.managedPaths. It is an array of package-manager only managed paths. webpack will avoid hashing/timestamping them, assume the version is unique and will use this as snapshot (for both memory and filesystem cache).
- These must be node_modules directories.
- cache.managedPaths defaults to the node_modules directory webpack is inside of.
add cache.buildDependencies. It is an object of arrays of additional code dependencies of the build. webpack will use an hash of these items and all dependencies to invalidate the filesystem cache.
- cache.buildDependencies.defaultWebpack defaults to webpack/lib to get webpack and all dependencies.
- It's recommended to set cache.buildDependencies.config: [__filename] in webpack.config.js to get the config and all dependencies.

Use schema-utils@2 for schema validation
- validateSchema API now throws an Error instead of returning an Array of Errors
Persistent Caching uses the infrastructureLogger for logging