<h2>What JetStream 3 is</h2>
JetStream 3 is a new browser benchmark suite built collaboratively by Apple, Google, Mozilla, and other partners. It replaces JetStream 2, which dates back to 2019 and was showing its age.
The core philosophy shift: instead of measuring isolated startup or runtime phases, JetStream 3 scores the full lifecycle of each workload - first iteration, worst case, and average case. That’s a more honest picture of how a browser actually behaves when running something real.
<h2>What’s in it</h2>
The suite focuses on computationally intensive workloads: browser games, physics simulations, cryptography, and complex algorithms. On the JavaScript side, new additions include React server-side rendering, TypeScript compilation, Babylon.js, Three.js, and MobX startup performance - things developers actually ship.
WebAssembly coverage expanded significantly. There are 12 new Wasm workloads covering Rust, Kotlin/Wasm, Dart2wasm, J2CL, and .NET toolchains. Wasm now makes up 15-20% of the suite, up from 7% in JetStream 2. That reflects where the web is actually heading.
The benchmark also runs in engine shells like V8’s d8, which makes iteration faster for browser teams.
<h2>What the WebKit team did with it</h2>
Apple’s WebKit team used JetStream 3 as a driver for a round of JavaScriptCore improvements. Some of the more interesting ones:
WasmGC allocation inlining. The compiler previously generated two allocations for certain GC patterns. Inlining eliminated that, producing roughly 40% gains on affected workloads.
New register allocator. The old allocator struggled with large functions. The replacement uses a greedy approach with B+ tree interval tracking and runs about 2x faster on large functions.
Polymorphic indirect call inlining. Wasm indirect calls through tables can now be inlined even when multiple target types are possible.
BigInt arithmetic. Comba multiplication and DIV2BY1 division were added, improving performance on cryptographic workloads.
Promise and microtask queue rewrite. The microtask queue and Promise combinators were rewritten from JavaScript builtins to C++. This sounds unglamorous but had a measurable impact on workloads that do a lot of async coordination.
GC type checking. Cohen’s type display algorithm was added for faster type checks, alongside an overhaul of GC type reference counting.
Collectively, these changes delivered roughly a 10% performance improvement from Safari 26.0 to 26.4.
<h2>Governance</h2>
JetStream 3 uses a consensus model across all major browser vendors. That’s worth noting - benchmarks developed by a single vendor tend to drift toward measuring things that vendor is already good at. Having Apple, Google, and Mozilla all involved in the design makes the results harder to game and more meaningful as a cross-browser comparison.

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JetStream 3 is a new browser benchmark suite developed collaboratively by Apple, Google, Mozilla, and partners, replacing the aging JetStream 2 from 2019. It measures the full workload lifecycle rather than isolated phases, and includes real-world workloads like React SSR, TypeScript compilation, Babylon.js, Three.js, and 12 new WebAssembly workloads covering Rust, Kotlin/Wasm, Dart2wasm, and .NET. Wasm now represents 15-20% of the suite. Apple's WebKit team used JetStream 3 to drive JavaScriptCore improvements including WasmGC allocation inlining (~40% gains on affected workloads), a new greedy register allocator with B+ tree interval tracking (2x faster on large functions), polymorphic indirect call inlining, BigInt arithmetic improvements, a Promise/microtask queue rewrite from JS builtins to C++, and faster GC type checking. These changes collectively delivered ~10% performance improvement from Safari 26.0 to 26.4. The benchmark's multi-vendor governance model makes results harder to game and more meaningful for cross-browser comparisons.

JetStream 3: what changed and why it matters for browser performance