Multithreading in JavaScript: A Guide to Web Workers

About this talk

Can Web Workers really solve your frontend performance bottlenecks? This talk at CityJS Athens 2025 explored the complexities of the JavaScript runtime — the event loop, asynchronous programming model, and single-threaded constraints — to find out if it is worth exploring the multithreaded world. Live demos showed how offloading heavy computation to a Worker thread keeps the main thread free and UIs smooth, along with an honest look at the trade-offs.

Key takeaways

• How the JavaScript event loop and async model relate to (and differ from) true parallelism
• When to reach for Web Workers — and when not to
• Communication patterns: postMessage, structured cloning, and Transferable objects
• Measuring before-and-after impact on frame rate and interaction latency