Why do HTML5 Web Workers have a high startup latency?

Question

According to an article by a Mozilla engineer Web Workers take about ~40ms to startup (latency).

How fast are Web Workers? https://hacks.mozilla.org/2015/07/how-fast-are-web-workers/

Web workers take about 40 ms to be instantiated. Also, this time is pretty stable with variations of only a few milliseconds.

In a JSFiddle test I notice a varying startup latency between 15ms to 70ms. The latency does not appear to be stable around ~40ms. Chrome appears to be faster (6ms to 25ms).

var workerCode = URL.createObjectURL(blob); // no download latency
var worker = new Worker(workerCode);

https://jsfiddle.net/pvr5xroh/1/

Why do Web Workers have such a high startup latency?

Answer 1

When you load a webworker like :

worker = new Worker('path/to/js');

The browser need to download 'path/to/js' before. Remember that JS is downloaded from the backend and executed in the frontend.