perf(optimizer): bulk optimizer delay

[sapphi-red]

Description

runOptimizerWhenIdle was called many times.
Also it was called serially every time waiting for .done() to be completed.

This PR reduces runOptimizerWhenIdle by calling multiple .done() in a single runOptimizerWhenIdle call. Also this PR calls .done() parallelly.

I ran this PR on https://github.com/sapphi-red/performance-compare/tree/65d51a32b0282593f1b75def25075a02b28f6397.
Without this PR (420782c76733ecc8314bccc2255976347add3d2e), start up time (with --force) was 16422ms with plugin-react.
With the PR, start up time was 4784ms. This is about 3.4x improvement.

I guess this improves performance if there's many files.

More detailed result of the benchmark: https://gist.github.com/sapphi-red/d1ee341566a052703732238788cb0447

Additional context

---

What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

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[patak-dev]

The setTimeout 0 here was added at the time because things were failing if we did a direct call. It may have been a bug that prevented us to do it. I think we can try it out without it now, let's keep in mind that we could add it later here if needed (I don't think it would affect perf)

[patak-dev]

It is incredible how much of a difference this makes. Awesome catch @sapphi-red! 🔥