As shown at <https://grafana-admin.wikimedia.org/dashboard/db/performance-metrics> with the % indicators, and more clearly at <https://graphite.wikimedia.org/render/?width=800&height=600&from=-8hours&target=legendValue(frontend.navtiming.{connecting,sslNegotiation,sending,receiving,loading,totalPageLoadTime}.overall.count,%22total%22,%22max%22)>, somewhere along the way data appears to be lost.
The number of events containing connectStart/End (aka "sending") doesn't nearly match the count of responseStart/responseEnd ("receiving"), and even more so for loadEventStart/End ("loading" and "totalPageLoadTime"). See also [puppet: navtiming.py](https://github.com/wikimedia/operations-puppet/blob/05b22833f8134406f38db92d2663cf39e87daa33/modules/webperf/files/navtiming.py#L75-L93).
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This may be related to the dropping of zero values (since later events seem to be favoured over earlier events, which would make sense if the earlier events all happen in <1ms or even 0ms if they don't happen due to re-using of connections or caching). See also https://phabricator.wikimedia.org/T104902#1626842 for several pending changes to be made.
Either way, tracking this separately as it seems unlikely zero values are the (only) cause of this (if at all).