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Patrick Meenan
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Chrome 41 shipped with several optimizations to make the visual experience of loading sites faster (Speed Index, Start Render, etc).  The most impactful of the changes was to have the main parser conditionally let pending paints and layouts happen before going into execute a script.

In practice this means that before running scripts at the end of a page it will do the layout and paint for whatever it has built up so far (assuming it is not waiting for css).  It doesn't change the classic technical metrics (Page Load Time, DOM Content Loaded, etc) but it can have a huge benefit to more visual-based metrics, particularly first paint/start render and Speed Index (and more importantly, on when users can see progress and start consuming the page content).

A nice side-effect is that it also improves performance for things like custom fonts and prioritization of above-the-fold resources, both of which depend on layout and styles being applied before they can be discovered by Chrome.

Here are some sample filmstrips with before/after comparisons demonstrating the impact (you may need to zoom in on some of the longer filmstrips to see them).

As always, your mileage may vary and not all sites will see the same magnitude of improvement (if any).
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2015-03-25
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That thing where you take the day off and still spend 1/2 the day working on stuff you'd normally be doing because it's what you enjoy.

 I'm still amazed they pay me to play with computers but I'll take it :-)

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Updated WebPagetest "Data Center" Tour
It has been 3 years since the last tour and a lot of people have been asking if it is still hosted in my basement so it's time for an update. First, yes it is still hosted out of my basement.  I did move it out of the utility room and into a storage room so...

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+Mike Pinkerton +Justin Cohen Ummm, eww

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New Android testing is now live on WebPageTest.  60FPS video capture, full timelines, nice selection of devices and coming soon as a private instance release.

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Android web performance testing takes a big leap forward
We've been working for a while to bring better support for measuring web performance on mobile for a while.   Michael Klepikov  started out by building out a new cross-platform test agent for WebPagetest that runs on Node.js, can run WebDriver/Selenium scri...

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I didn't realize that Mozilla made their Firefox field performance stats public.  Lots of awesome RUM data at http://telemetry.mozilla.org/

Here are some of my favorites for using as a baseline for your own RUM metrics (setting goals, etc).  Make sure to change the preferences to show evolution over calendar dates and un-select any percentile that has a huge spike to make the graphs cleaner:

Page Load Time: http://telemetry.mozilla.org/#release/26/FX_PAGE_LOAD_MS
Median is 1.3 seconds and the 95th percentile is 10 seconds (actually a lot better than I expected)

DNS Lookup Time: http://telemetry.mozilla.org/#release/26/DNS_LOOKUP_TIME
Median is 45ms but the 95th percentile is 660ms - that's scary-long.

The RTT measured from the SPDY layer (couldn't find the TCP equivalent but it may be in there): http://telemetry.mozilla.org/#release/26/SPDY_SETTINGS_RTT 104ms median and 473ms 95th percentile.  Could be lower than the DNS numbers since SPDY is dominated by large sites right now (Google, Facebook, Twitter).

+Patrick McManus Do you know if the TCP socket connect time is in there anywhere?  That is usually a great proxy for the RTT.

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There are 2 pretty big #webperf  changes to Chrome's resource preloader that are now live in Canary and should be coming out to stable when 33 launches:

1 - The preload scanner now scans more of the document and "sees" more resources than it did before: https://code.google.com/p/chromium/issues/detail?id=284927

2 - As the browser does layout for images, it should boost the priority of visible images: https://code.google.com/p/chromium/issues/detail?id=319073

If you're doing any fancy trickery to try to get browsers to load things in a specific order it's worth testing with Canary to make sure your techniques still work (and if you're not it's still worth testing to see if the changes help your site).

If you see anything really bizarre or unexpected let us know: http://crbug.com/new (or more informally just comment here if you have a WebPagetest waterfall from Chrome Stable and Canary that you want me to look at).

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Yeah, you could say we enjoy Halloween :-)

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