Webpack 5 triggers a slow V8 concat path resulting in significant performance degradation in some scenarios

[eoin]

Early in Webpack 5 development the arrayToSetDeprecation helper was added to support migrating some objects from Array to Set. In order to maintain Array-like behaviour in concat the helper sets Symbol.isConcatSpreadable. On V8, setting Symbol.isConcatSpreadable anywhere puts all subsequent invocations of concat within an Isolate on a slow path.

There are cases where this can result in significant performance degradation. In this repro case, builds go from 1s with Webpack 4 to 16s with Webpack 5. We came across this when debugging this Linaria issue, where our production build slowed from ~10s to 2m+. I wonder if this could be the cause of some issues in #12102.

[sokra]

Oh interesting...

I wanted to add an option to opt out of deprecations anyway. Something like experiments.webpack4Compat: false. And they will be removed in webpack 6. So another reason to release that sooner than later...

[eoin]

For anyone who might be interested we did a writeup of how we tracked down this issue.

[zloirock]

@eoin deprecated versions of core-js (version < 3.3) cause the same problems, now about 20-25% (a little obsolete results of naive detection) of TOP 10000 websites use those deprecated core-js versions. It's @@isConcatSpreadable and (mainly) @@species feature detection, even without polyfilling. Workarounds of those V8 whims were added 2+ years ago, but who read deprecation notes?