SGTM! Timeout: the average reading speed is ~180 words per minute (https://goo.gl/EH36iT), so users need around 2000ms to read the message (~5-6 words depending on the translation). Adding another 2000ms time for the cognitive context-switch, I would suggest 4000ms for the minimum duration the message is shown. For the threshold I think it makes sense use the 140ms we determined for the data counter message (“Calculating…”). 

Resulting behavior: show the dialog only after 140ms and show it for at least 4000ms.

WDYT?

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I would be a bit careful about applying the counter behavior so straightforwardly here. Counters are a secondary feature, and you don't have to wait for them to finish. Deletion is the primary feature, and it's blocking. I'm not sure if making the product slower so that it's visually more pleasing is a good idea.

I'm open to try it, but please let's add a histogram for an "unnecessarily shown dialog", i.e. for the deletion time 140-4000.

Alternatively, note that we changed the MD CBD on Desktop to not wait for deletion (https://codereview.chromium.org/2315443002), which seems to have worked fine so far, although MD settings have been behind a flag most of the time, so we probably don't have enough data yet to tell if people mind that data are still being deleted while they browse.

That's a good point. Avoiding any blocking UI would be preferable over this option. Are there any technical differences on Android that would prevent us from adapting the Desktop behavior?

It could be an issue if you open the CBD dialog from history and select to delete all your history. When you click "Clear Data", you will go back to the history dialog but it might still show history entries if the deletion didn't finish?

The backend is the same, so this will work on Android as well. But this is a somewhat fundamental change in behavior, so I wouldn't recommend merging it.

Given that this is not a regression, can we make the change one milestone later?

Sounds good! This will give us some time to think about other non-blocking options like snackbars. Should we still add the histogram to learn more about deletion times in the meantime?

Issue 801241 has been merged into this issue.