Make `detachDeletedInstance` 30% faster

[MrFlashAccount]

According to how JS works, null instead of delete keyword is faster from 2x to 4-5x depending on a browser engine.

I reviewed the usage of these fields across the codebase and everywhere we check if the key is undefined or null, so the change is safe. All tests passed.

The main benefit - it speeds up user interactions, because (at least in my case) this method runs synchronously when I type in an input.

Summary

• Replace uses of delete obj[key] with obj[key] = null in the detachDeletedInstance function
• Motivation: delete changes object shape (hidden class) and deoptimizes inline caches.
• Semantics: I reviewed the usage of the keys, we don't do key in obj for these keys across the codebase.
• No public API changed (given the keys are not a part of the public api)

On Chrome/Firefox/Safari locally, the null-assignment loop was consistently faster (roughly 2–5x depending on engine/version), matching expectations from hidden-class/IC behavior.

How did you test this change?

• yarn test
• tests/perf benches in the chrome devtools, confirming the improvement
• All tests passed locally.

Comparison

Methodology:

• React in prod mode
• Without React devtools
• 6x slowdown in chrome devtools
• Macbook Pro M1 max

Before:

After:

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[MrFlashAccount]

Maybe someone from the react core team could take a look at this PR? @eps1lon , @rickhanlonii , @acdlite ? 🙏