10 Key Insights into JetStream 3: The New Cross-Browser Benchmark Suite

When browser engine developers talk about performance, benchmarks are often their best toolbox. But as the web evolves, even the most trusted benchmarks can grow stale. That's where JetStream 3 comes in—a major update announced jointly by Apple, Google, and Mozilla. This new suite isn't just a refresh; it's a fundamental shift in how we measure browser performance, especially for WebAssembly and modern web applications. Let's dive into the ten most important things you need to know about JetStream 3.

1. A Tri-Company Collaboration

JetStream 3 isn't the work of a single team. It's the result of a shared effort between the WebKit team (Apple), V8 team (Google), and SpiderMonkey team (Mozilla). This collaboration ensures that the benchmark reflects the priorities of all major browser engines, making it a fair and reliable tool for performance measurement across Chrome, Safari, and Firefox.

2. WebAssembly Takes Center Stage

One of the biggest changes in JetStream 3 is how it handles WebAssembly (Wasm). In JetStream 2, Wasm was still emerging, and the benchmark focused on large C/C++ applications with heavy startup costs. Now, Wasm is used everywhere—from image decoders to UI frameworks—so JetStream 3 reworks its Wasm subtests to reflect this broader, more critical role.

3. The "Zero Startup Time" Problem

Browser engines have become so efficient that for small Wasm modules, startup time can effectively be zero milliseconds. In JetStream 2, this created a mathematical nightmare: a score computed as 5000 divided by time, where a zero time yields infinite score. WebKit had to patch in a clamp of 5000 to keep scores sensible. JetStream 3 eliminates this issue entirely with a new scoring approach.

4. A New Scoring Formula for Wasm

Rather than measuring startup and runtime separately (as in JetStream 2), JetStream 3 blends these phases into a single, realistic workload. The benchmark now runs Wasm tasks that include both instantiation and execution, then calculates a geometric mean across subtests. This prevents the infinity problem and gives a score that reflects real-world performance.

5. Focus on Micro-optimization Traps

When benchmarks get too focused on small workloads, browser engineers might chase micro-optimizations that don't benefit real pages. JetStream 3's Wasm tasks are designed to be representative of actual web usage—like decoding an image or running a library function—so that optimizations translate to real user gains, not just benchmark wins.

6. Larger and More Complex Workloads

The web is growing in scale, and JetStream 3 reflects that. New tests involve larger data sets, more concurrent tasks, and realistic application patterns. For example, JavaScript workloads now include larger module graphs and more complex asynchronous operations, pushing engines beyond simple loop optimization.

7. Rethinking the Balance of Startup vs. Throughput

JetStream 2 separated startup and throughput scores, but modern Wasm often blurs that line. Libraries that previously took time to load are now instantiated in the critical path of page loads. JetStream 3 treats Wasm as an integrated part of the application, measuring how quickly the entire task—from load to result—finishes.

8. A Broader Set of Real-World Tests

Beyond Wasm, JetStream 3 includes new JavaScript benchmarks that mimic modern frameworks, data processing, and animation. These tests were sourced from real-world libraries and applications, ensuring that optimizations that improve benchmark scores also speed up actual websites.

9. Backward Compatibility, But Different Scoring

While JetStream 3 is a major update, it maintains some legacy subtests from JetStream 2 to allow cross-version comparisons. However, the overall scoring methodology has changed—now using a geometric mean of all subtests rather than a sum or weighted average—making direct score comparisons with earlier versions less meaningful.

10. Open Source and Community Driven

JetStream 3 is fully open source, hosted on GitHub. Developers from any browser project can contribute new tests or suggest modifications. This transparency helps ensure the benchmark stays relevant as the web platform evolves, and it encourages a community-wide focus on performance improvements that benefit everyone.

JetStream 3 represents a thoughtful evolution in browser benchmarking. By addressing the peculiarities of WebAssembly, modernizing workloads, and fostering cross-engine collaboration, it sets a new standard for measuring performance. Whether you're a browser engineer, a web developer, or just curious about what makes your browser fast, JetStream 3 offers a clearer, more honest picture of today's web capabilities.