In analyzing metrics reported from the DelayNavigationPageLoadMetricsObserver, I discovered that there are 25% fewer metrics reported for this observer than for the core observer, for metrics recorded in OnFirstPaint with what is supposed to be the same logging policy.

One key difference in policy is that the DelayNavigationPageLoadMetricsObserver stops observing when OnHidden is invoked, whereas the core observer continues to observe.

It's possible that for page loads where we encounter FCP then get backgrounded, we fail to record metrics in DelayNavigationPageLoadMetricsObserver, as the FCP IPC may be delayed by up to 1 second.