Web Workers allow for long-running scripts that are not interrupted by scripts that respond to clicks or other user interactions and allow long tasks to be executed without yielding to keep the page responsive.
The Web Workers specification defines an API for spawning background scripts in your web application. Web Workers allow you to do things like fire up long-running scripts to handle computationally intensive tasks, but without blocking the UI or other scripts to handle user interactions.
REST APIs are naturally multi-thread, once they can execute multiple requests at the same time. Therefore, every time you put a thread to wait for something synchronously you are wasting CPU time because that thread could be being used to handle another request.