Breaking: Major Browser Vendors Release JetStream 3.0
Apple, Google, and Mozilla today jointly announced JetStream 3.0, a sweeping update to the cross-browser benchmark suite that fundamentally changes how web performance is measured. The new benchmark addresses critical flaws in its predecessor, particularly around WebAssembly (Wasm) workloads that had led to mathematically infinite scores.
"JetStream 2 had reached its limits — we were scoring infinity on some subtests because startup times hit zero milliseconds," said a WebKit engineer familiar with the development. "That's not useful for driving real-world improvements."
Key Changes: WebAssembly Gets a Ground-Up Redesign
The most significant overhaul involves WebAssembly benchmarking. JetStream 2 measured Wasm in two separate phases — Startup and Runtime — but modern engines have become so fast that startup times collapsed below measurable thresholds.
"When we optimized the startup path in JavaScriptCore, certain smaller workloads reached zero seconds per iteration," the engineer explained. "The scoring formula Score = 5000 / Time then produced infinity, which required a patch in JetStream 2.2 to clamp the score at 5000."
JetStream 3 eliminates this problem by redesigning Wasm subtests to reflect current usage patterns, where Wasm is often in the critical path for page loads — used in libraries, image decoders, and UI frameworks.
Background: Why a New Benchmark Was Urgent
Benchmarks are essential tools for browser engine developers, but they must evolve with the web. JetStream 2, released years ago, was based on an era when WebAssembly was in its infancy and large C/C++ projects tolerated long startup costs for high throughput.
"The web has moved on," said a Google V8 team member. "Wasm is now used everywhere — from video games to productivity apps. A benchmark that rewards zero startup time without considering real-world application behavior isn't helping anyone."
The collaboration among Apple (WebKit), Google (V8), and Mozilla (SpiderMonkey) ensures that JetStream 3 reflects a consensus on what matters for modern web performance.
What This Means for Browser Performance
For developers and users, JetStream 3 means benchmarks will better predict actual browsing experiences.
"Previously, optimizing for JetStream 2 could lead to micro-optimizations that didn't translate to real-world gains," noted a senior engineer at Mozilla. "With JetStream 3, the incentives align with what users actually encounter: faster page loads, smoother interactions, and efficient Wasm execution."
The update also reflects a broader shift: WebAssembly is now a first-class citizen in browser performance measurement, not an afterthought. The new suite includes a refreshed set of workloads that mirror modern web applications at scale.
Immediate Implications
- Browser vendors will prioritize optimizations that matter for JetStream 3, potentially accelerating Wasm startup improvements for real applications.
- Web developers can use JetStream 3 as a more reliable proxy for end-user performance, especially on Wasm-heavy sites.
- Competitive benchmarking will see a reset; scores from JetStream 2 cannot be directly compared with the new version.
As the web continues to evolve, JetStream 3 sets a new baseline — one that doesn't break when engines get too fast. The three major vendors are committed to periodic updates, with the next version already under discussion.
"This isn't the end," the WebKit engineer concluded. "It's the beginning of a more honest conversation about performance."