The browser, iOS, and Android all use a main thread separate from the thread responsible for scrolling animations. However, I think it's true that controlling

The browser, iOS, and Android all use a main thread separate from the thread responsible for scrolling animations. However, I think it's true that controlling threading in order to perform gnarly work in a separate thread is more ergonomic on mobile than in JS/React. You'd need to create a Promise that wraps a Web Worker, which would be an unusual thing to use. I don't think most apps need such control over threading in the browser.