Paul Paul - 6 months ago 16
HTML Question

Understanding Firebug's Net panel

I am trying to get a hang on analysing the performance of a web page using Firebug's Net panel.

The following screenshot shows an example of a google query. For the sake of this discussion I clicked twice, so some requests are cached.

Screenshot of Firebug's *Net* panel

So here are my questions:

1) What is happening between the end of the first request and the beginning of the next request (which is the third one). In the same context: Why is the third request starting earlier than the second request?

2) The next 6 requests are coming from the cache. The purple bar is indicating waiting time and I assumed this is the time the browser "waiting for the server to to something". So as comes from cache, what exactly is the browser waiting for. Also: What could be the reason, that the waiting time for for a 4,4KB response is longer (63ms) than for a 126,3 KB response (50ms).

3) In the next request there is a fairly long green bar indicating the time of receiving the response. How comes that this doesn't seem to be at least fairly proportional to the size of the response?

4) The red vertical line indicates the load event. According to https://developer.mozilla.org/en-US/docs/Web/Events/load this means: "The load event is fired when a resource and its dependent resources have finished loading." In the timeline you can see that there are still a couple of requests performed after the load event. How comes? Are they considered to be not dependent and if so why?

Answer
  1. The response of the first request needs to be parsed to find out what else needs to be loaded. This parsing time causes the gap to the second request. See also my answer to a related question.

  2. Responses coming from cache still have an associated network request, which returns the 304 HTTP status code. You can see the request and response headers as well as the response headers of the cached response when you expand the request.

    In contrast to that there is also a response that is directly served from a special cache called Back-Forward Cache (or BFCache). These responses happen directly after the browser start when you have the option enabled to restore your tabs from the last session and also when you navigate back and forth in the tab history.

  3. This depends on the network connection speed and the response's size in the first place but also on how long the server takes to send the full response. Why that one request takes that long in comparison to the others can't be explained without knowing what happens on the server side.

  4. The load event is fired when the page request is loaded including all its depending resources like CSS, images, JavaScript sources, etc. Requests initiated after the load event are loaded asyncronously, e.g. through an XMLHttpRequest or the defer attribute of the element.

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