For all resources, add timestamp of when it was available to the browser

What happens

There are cases when a resource fetched over the network has completed downloading, and is available to the browser, but the browser is too busy to read it and parse it. In these cases, the responseStart timestamp is set after the browser becomes free to read the file off its buffer. This incorrectly points to a network problem.

Example

As an example, using WebPageTest, we measured the loading of a particular website, and also captured the network PCAP file of the data transfer. The PCAP file showed that the file had been fetched (all packets arrived at destination) in under 200ms, however the browser reported responseStart at the 1.2second mark. WPT showed that the browser was at 100% CPU utilization and 0% bandwidth utilization during these 1.2 seconds. This indicated that while the file was available locally in some buffer, the browser was too busy to read it, and incorrectly put the blame on the network.

What we'd like to have

1. responseStart should stamp when bytes of the response were available, not when the browser was ready to parse them
2. responseEnd should stamp when the response was completely available, not when the browser finished parsing
3. Add a new timestamp called loadComplete or resourceLoaded or equivalent, indicating when the browser actually finished handling the resource in a way appropriate for that resource (which might include inserting into the DOM, parsing, executing, etc.)

I believe 1 & 2 are mostly in line with the spec, we should make it explicit that the browser being unable to load, parse and execute an available resource should not delay the responseStart and responseEnd timestamps, and browsers need to follow this.

Source Discussion

This came up as part of the W3C discussion at the Velocity Conference in Santa Clara, CA on 2016-06-22T19:00PDT.

Members in attendance during discussion: (please update list in case I've missed/misspelt anyone)

• Ryosuke Niwa
• Ilya Grigorik
• Philip Tellis

Start by reviewing the Resource Timing behavior described for responseStart and responseEnd, then read the linked source discussion context in this issue. The work is complete only when the timing semantics and the proposed loadComplete or resourceLoaded timestamp have an agreed specification-level definition.