The sync IPC is required in order to get a routing id assigned by the browser process. The content::RenderFrame needs to be created for the iframe with a valid routing id, so we can't do this async. What is interesting to me is that the createChildFrame is overlapping the didCommitProvisionalLoad. Did you add a frame with no source or something else?
The commit will depend on the network, but should be synchronous for about:blank.

When we create the frame, we synchronously commit the initial empty document: that's where that's coming from. I guess there's some subsystems doing a bunch of work in that callback...

esprehn confirmed to me that the slow didCommitProvisionalLoads are coming from about:blanks.

I'm trying to repro now with some added probe points. There have been a bunch of places where we add crash keys to learn about failed CHECKS. I wonder if those are contributing to the slowness if the CHECK succeeds (an alternative would be to CHECK_NOTREACHED() after an if so we only add crash keys on a known failure).

I was able to repro the slowness (~6ms didCommit*) on Chrome stable 52.0.2743.98 but could *not* repro on Chrome Dev (54.0.2837.2) or TOT (@414688). This was on a Pixel C. These methods were taking < .5 ms. 

Unfortunately, this means that adding probe points doesn't really help. Does anyone know if there were optimizations in this area since M52?

Pls don't use Needs-Feedback for anything other than flagging bugs that need additional reporter (user of chrome) input.

Has anyone reprod the slowness with a build > M52?

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This issue has been Available for over a year. If it's no longer important or seems unlikely to be fixed, please consider closing it out. If it is important, please re-triage the issue.

Sorry for the inconvenience if the bug really should have been left as Available.

For more details visit https://www.chromium.org/issue-tracking/autotriage - Your friendly Sheriffbot