We can rule out WebPageTest changes and Chrome changes because we get the exact same behavior on my test instance for all article pages that aren't a stub (where we use the same Chrome version and no WPT):

Will look into https://www.mediawiki.org/wiki/MediaWiki_1.29/wmf.10 and https://wikitech.wikimedia.org/wiki/Server_Admin_Log during the day.

The change is pretty remarkable in domInteractive, we can see that in the Navigation Timing data, we got an improvement there:

And before and after for WebPageTest the domInteractive has gone from 1.68s to 0.54s and the compare between runs but that also generated the regression in SpeedIndex and startRender. Ping @Krinkle please have a look when you have time.

That incredible jump in domInteractive for WebPageTes could be comparing one high with one low. Looking at it over time, it shows decreased values since Feb 3:

Four runs compared, two before and two after: http://wpt.wmftest.org/video/compare.php?tests=170208_NW_MN,170206_2D_6G,170201_SG_3K,170201_9W_E

One thing that I missed is that things looks better for Firefox after the change, at least it visible for all tests we do for the Facebook page:

SpeedIndex Second view

SpeedIndex first view

SpeedIndex authenticated

I've been comparing the HTML inside of head to see if there's some difference before and after, and there's one for both Facebook and Chrome the og:image property was added in the latest release:

<meta property="og:image" content="https://upload.wikimedia.org/wikipedia/en/b/bf/Facebook_user_page_%282014%29.jpg" />

and

<meta property="og:image" content="https://upload.wikimedia.org/wikipedia/commons/thumb/5/56/Donald_Trump_official_portrait.jpg/1920px-Donald_Trump_official_portrait.jpg"/>

but shouldn't do anything with the performance or ...?

This is more interesting, we used to have an inline script at the beginning of the first content that was removed in the latest release:

<h1 id="firstHeading" class="firstHeading" lang="en">Facebook</h1>
<div id="bodyContent" class="mw-body-content">
            <div id="siteSub">From Wikipedia, the free encyclopedia</div>
            <div id="contentSub"></div>
            <div id="jump-to-nav" class="mw-jump"> Jump to: <a href="#mw-head">navigation</a>, <a href="#p-search">search</a> </div>
            <div id="mw-content-text" lang="en" dir="ltr" class="mw-content-ltr">
                <script>
                    function mfTempOpenSection(id) {
                        var block = document.getElementById("mf-section-" + id);
                        block.className += " open-block";
                        block.previousSibling.className += " open-block";
                    }
                </script>