Significant regression in dev watch mode performance

[MLoughry]

Bug report

What is the current behavior?
We recently upgraded from webpack 4.6.0 to 4.12.0, and found our watch mode build times balloon from 120s initial/17s rebuild to 250s initial/150s rebuild.

I used git bisect to determine that the regression was caused by #7373, which is a single-line change to clone chunkGroups arrays before processing them. Reverting this resolves the build time regression.

If the current behavior is a bug, please provide the steps to reproduce.
I don't have an easy repro, as our repository is quite large and can't be shared out. @TheLarkInn may be able to assist, since he would have access to our repository.

What is the expected behavior?
Not to regress dev watch build times

Other relevant information:
webpack version: 4.9.0
Node.js version: v8.11.1
Operating System: Windows 10
Additional tools:

[MLoughry]

@sokra, there was no explanation included in #7373 or its linked bugs as to how the change was meant to fix the require.ensure() issue, and I'm not familiar enough with the code to understand. Obviously I can't simply revert that change, as it breaks your new tests and regressed those bugs; but I have no idea of another mechanism to fix it while fixing this perf regression.

[MLoughry]

I did a bit more investigation, but I can't say it makes sense. The call to .slice() is, as expected, very quick. However, the call to this.processDependenciesBlocksForChunkGroups is significantly slower when given a cloned array, rather than the original.

I'll try to dig deeper later.

[MLoughry]

I've temporarily worked around the issue in our build system by monkey-patching webpack to revert the offending commit, since the bug it fixed never affected us.

[webpack-bot]

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It's subject to automatic issue closing if there is no activity in the next 15 days.

[webpack-bot]

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