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…yload (#35776) ## Summary Follow-up to vercel/next.js#89823 with the actual changes to React. Replaces the `JSON.parse` reviver callback in `initializeModelChunk` with a two-step approach: plain `JSON.parse()` followed by a recursive `reviveModel()` post-process (same as in Flight Reply Server). This yields a **~75% speedup** in RSC chunk deserialization. | Payload | Original (ms) | Walk (ms) | Speedup | |---------|---------------|-----------|---------| | Small (2 elements, 142B) | 0.0024 | 0.0007 | **+72%** | | Medium (~12 elements, 914B) | 0.0116 | 0.0031 | **+73%** | | Large (~90 elements, 16.7KB) | 0.1836 | 0.0451 | **+75%** | | XL (~200 elements, 25.7KB) | 0.3742 | 0.0913 | **+76%** | | Table (1000 rows, 110KB) | 3.0862 | 0.6887 | **+78%** | ## Problem `createFromJSONCallback` returns a reviver function passed as the second argument to `JSON.parse()`. This reviver is called for **every key-value pair** in the parsed JSON. While the logic inside the reviver is lightweight, the dominant cost is the **C++ → JavaScript boundary crossing** — V8's `JSON.parse` is implemented in C++, and calling back into JavaScript for every node incurs significant overhead. Even a trivial no-op reviver `(k, v) => v` makes `JSON.parse` **~4x slower** than bare `JSON.parse` without a reviver: ``` 108 KB payload: Bare JSON.parse: 0.60 ms Trivial reviver: 2.95 ms (+391%) ``` ## Change Replace the reviver with a two-step process: 1. `JSON.parse(resolvedModel)` — parse the entire payload in C++ with no callbacks 2. `reviveModel` — recursively walk the resulting object in pure JavaScript to apply RSC transformations The `reviveModel` function includes additional optimizations over the original reviver: - **Short-circuits plain strings**: only calls `parseModelString` when the string starts with `$`, skipping the vast majority of strings (class names, text content, etc.) - **Stays entirely in JavaScript** — no C++ boundary crossings during the walk ## Results You can find the related applications in the [Next.js PR ](vercel/next.js#89823 I've been testing this on Next.js applications. ### Table as Server Component with 1000 items Before: ``` "min": 13.782875000000786, "max": 22.23400000000038, "avg": 17.116868530000083, "p50": 17.10766700000022, "p75": 18.50787499999933, "p95": 20.426249999998618, "p99": 21.814125000000786 ``` After: ``` "min": 10.963916999999128, "max": 18.096083000000363, "avg": 13.543286884999988, "p50": 13.58350000000064, "p75": 14.871791999999914, "p95": 16.08429099999921, "p99": 17.591458000000785 ``` ### Table as Client Component with 1000 items Before: ``` "min": 3.888875000000553, "max": 9.044959000000745, "avg": 4.651271475000067, "p50": 4.555749999999534, "p75": 4.966624999999112, "p95": 5.47754200000054, "p99": 6.109499999998661 ```` After: ``` "min": 3.5986250000005384, "max": 5.374291000000085, "avg": 4.142990245000046, "p50": 4.10570799999914, "p75": 4.392041999999492, "p95": 4.740084000000934, "p99": 5.1652500000000146 ``` ### Nested Suspense Before: ``` Requests: 200 Min: 73ms Max: 106ms Avg: 78ms P50: 77ms P75: 80ms P95: 85ms P99: 94ms ``` After: ``` Requests: 200 Min: 56ms Max: 67ms Avg: 59ms P50: 58ms P75: 60ms P95: 65ms P99: 66ms ``` ### Even more nested Suspense (double-level Suspense) Before: ``` Requests: 200 Min: 159ms Max: 208ms Avg: 169ms P50: 167ms P75: 173ms P95: 183ms P99: 188ms ``` After: ``` Requests: 200 Min: 125ms Max: 170ms Avg: 134ms P50: 132ms P75: 138ms P95: 148ms P99: 160ms ``` ## How did you test this change? Ran it across many Next.js benchmark applications. The entire Next.js test suite passes with this change. --------- Co-authored-by: Hendrik Liebau <mail@hendrik-liebau.de>
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