Web Workers handling AJAX calls - optimisation overkill?

I have created a proper benchmark for that on jsperf. Depending on the browser, WebWorker approach is 85-95% slower than a raw ajax call.

Notes:

• since network response time can be different for each request, I'm testing only new XMLHttpRequest() and JSON.parse(jsonString);. There are no real AJAX calls being made.
• WebWorker setup and teardown operations are not being measured
• note that I'm testing a single request, results for webworker approach may be better for multiple simultaneous requests
• Calvin Metcalf explained to me that comparing sync and async on jsperf won't give accurate results and he created another benchmark that eliminates async overhead. Results still show that WebWorker approach is significantly slower.
• From the Reddit discussion I learned that data passed between the main page and WebWorker are copied and have to be serialized in the process. Therefore, using WebWorker for parsing only doesn't make much sense, data will have to be serialized and deserialized anyway before you can use them on the main page.

First thing to remember is that web workers rarely make things faster in the sense of taking less time, they make things faster in the sense that they off load computation to a background thread so that processing related to user interaction is not blocked. For instance when you take into account transferring the data, doing a huge calculation might take 8 seconds instead of 4. But if it was done on the main thread the entire page would be frozen for 4 seconds which is likely unacceptable.

With that in mind moving just the ajax calls off the main thread won't gain you anything as ajax calls are non blocking. But if you have to parse JSON or even better, extract a small subset out of a large request then a web worker can help you out.

A caveat i've heard but not confirmed is that workers use a different cache than the main page so that if the same resources are being loaded in the main thread and the worker it could cause a large duplication of effort.

You are optimizing your code in the wrong place.

AJAX requests already run in a separate thread and return to the main event loop once they fulfil (and call the defined callback function).

Web workers are an interface to threads, meant for computationally expensive operations. Just like in classical desktop applications when you don't want to block the interface with computations that take a long time.