[v2] Perf: Use one atom subscription per Field - #2358

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You mention performance for the PR. Do you have some rough numbers for it? Memory / runtime?

@LeCarbonator all the perf is just from going from 2 to 1

I ran a focused benchmark using the exact before/after hooks with production React 19.2.8. Each variant ran in a fresh process 10 times, alternating order; results are medians.

1,000 mounted fields

The harness mounts non-array fields backed by individual TanStack Store atoms, measures synchronous mount/update work, and measures retained heap after forced GC.

This isolates subscription overhead rather than measuring a complete application, so the absolute timings are directional. The consistent result is half the subscriptions and callbacks, with roughly 3.2 KiB less retained heap per mounted field.

Sorry, don't have a ton of cycles here to respond more in depth, but I'd attempted this in v1 and benchmarks showed MASSIVE improvements then (like 10x), but the UX of 1000 mounted fields slowed to a crawl for reasons I couldn't identify.

Might be helpful for us to all sit down and discuss and on our end manually test.

Not saying that this is a bad PR or approach, just flagging that benchmarks for this has been directionally wrong before.

Let sol on ultra try to figure out what you were referring to, obviously feel free to review/close whatever.

Model used: OpenAI Codex — GPT-5.6 Sol

Codex investigated the relevant v1 history and benchmarked the exact base and PR commits in production Chromium.

The closest v1 match is PR #2036. That implementation replaced seven subscriptions with one selector that created a new seven-property object and used a custom comparator. It was also mixed with substantial core store and notification changes, so the historical results do not isolate subscription consolidation as the cause. Its “1,000-field” benchmarks actually rendered 100 fields in jsdom and did not measure browser rendering or memory.

V2 has a different design. Before this PR, a normal field subscribes separately to state.value and state.meta. The PR subscribes once to the existing {value, meta} snapshot. V2 core preserves that snapshot’s identity whenever both references are unchanged:

if (prev?.meta === meta && prev.value === value) {
  return prev
}

Therefore, the rerender conditions are equivalent:

before: value changed OR meta changed
after:  field snapshot changed

The PR does not create a projected object, add a custom comparator, or change core notification behavior. ArrayField is also untouched.

Real-browser confirmation with 1,000 controlled fields and 60 fresh contexts per variant found:

• Mount: 7.5% faster
• Typing to commit: 12.2% faster
• Typing p95: 12.7% faster
• JavaScript heap: 15.4% lower

@scttcper I'm an idiot and I wasted your tokens.

I just reviewed the diff quickly. For some reason I'd gotten a LOT wrong in my mind about what this PR was and assumed it was doing something more closely related to splitting one store into many for modularity. Frankly I don't know how I got things this mixed up.

So sorry.

This PR LGTM broadly, but might have weird consequences with Compiler. So long as it works well there we should merge.

Sorry again, that's embarrassing.

@crutchcorn work pays for it 🤷

I think this should be compiler safe. Don't see how changing it from two to one could have consequences for it.

I wish I had better Wi-Fi to actually try this, but that'll have to wait until next monday.

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☁️ Nx Cloud last updated this comment at 2026-08-27 21:06:00 UTC

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