I did a couple fast tests yesterday with the only pages I know that implements user timings for content and images: webpagetest.org and www.sitespeed.io.

When I look in WebPageTest "The time to text" always seems off (or at least not showing the exact same numbers) but the the time to first image looks better, but I haven't tested it fully in different browsers and connection speeds.

Let put this on hold. We tried out user timings for measuring image time and it worked out bad: T115600

The Chrome team is planning to include SpeedIndex in the browser: https://groups.google.com/a/chromium.org/forum/#!topic/loading-dev/E9FIy0B2AkU

In the webperformance slack channel Pat Meenan said that they plan to also make it reachable from JS (when it's battle tested). That would be great for us.

Got some updates: It seems the Chrome team stopped working on SpeedIndex but the w3c working group has a proposal for firstMeaningfulPaint (images/text/svg). This is implemented in Chrome event logs but not availible in JS yet. Firefox also have intentions to implement it https://bug635044.bugzilla.mozilla.org/show_bug.cgi?id=1298381 (a lot interesting links in that issue).

This is the issue we should keep track of: https://github.com/w3c/charter-webperf/issues/32

But for us I think the current firstPaint will be the same as the firstManingfulPaint. Checkout https://github.com/tdresser/time-to-first-paint/blob/master/README.md and for us in Chrome (at least) the page render like this:

First there's nothing:

And then the first real paint:

First image:

Logo:

(Removed #Upstream tag as this has been unblocked, Chrome released it, see updated task description.)

I think this still doesn't work as expected in Chrome (it's released I know, but ...) see:
https://codereview.chromium.org/2967733002/
https://chromium-review.googlesource.com/c/576371