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Start render and SpeedIndex is too optimistic for second view
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Description

We have a concept where we test the second page view (not repeat view) where we access one page and then measure the next page we visit. It's a important metric since we then can see how the browser cache/local storage will affect metrics.

We have had problems with it before when WebPageTest report too fast metrics and we have that again.

The TTFB is too fast 0.036s and the start render is completely wrong: 0.119s. Checking the filmstrip view we can see that start render happens after 2 seconds. Checkout this example:
http://wpt.wmftest.org/result/160312_53_4S/

We had the problem before with the WPT register start render when the Chrome login popup changed but that is fixed. Need to check again what's wrong, I though did a fix where we make the screen white before we do the second page view (to make the filmstrip view and video cleaner) but that seems to be gone now because the filmstrip view starts with the first page view.

Event Timeline

It's interesting that the RUM First Paint is a much better metric (using the first paint reported by Chrome). That metric always seems to get a lot of bashing from people, but for us it works good.

Change 277724 had a related patch set uploaded (by Phedenskog):
Clear the screen between runs to make SpeedIndex correct.

https://gerrit.wikimedia.org/r/277724

Change 277724 merged by jenkins-bot:
Clear the screen between runs to make SpeedIndex correct.

https://gerrit.wikimedia.org/r/277724

The clear the screen fix helped start rendering:

Screen Shot 2016-03-17 at 10.34.23 AM.png (510×2 px, 97 KB)

The SpeedIndex looks the same though but that maybe is right? I though it was calculated using start rendering, will check the docs and create an issue.

Did a quick check and we got strange (to fast) TTFB. I've created an issue: https://github.com/WPO-Foundation/webpagetest/issues/615

Krinkle triaged this task as Medium priority.Aug 11 2016, 7:23 AM
Krinkle removed a project: Patch-For-Review.

I've got info that this should be fixed when I was on vacation but when I briefly looked at the graph early this week it seems like it is still there, let me check it again later this week to be sure.

It actually looks good now :)