Although the performance of today’s JavaScript engines is sufficient for most web applications, faster and more predictable runtimes could be desired for performance-critical web code. Therefore, we present Speedy.js, a cross-compiler that translates JavaScript/TypeScript to WebAssembly, a new standard for native execution supported by all major browsers. Speedy.js only imposes minimal restrictions on the JavaScript code, namely that the performance-critical functions are wrapped in TypeScript and only engage a performance-optimal subset of the JavaScript language. With this approach, we manage to make compute-intense web code up to four times faster, while reducing runtime fluctuations to the half.