measure deltas between timing events

[sbeam]

This issue is more to get your thoughts on this as a feature.

We are seeing very data coming into our InfluxDB collector that is so noisy as to make it near impossible to get actionable metrics. For instance, domInteractive on a certain page has a mean of 8329 with a standard deviation of 13898. (sample size=62000+). Which is due to some very large long-tail response times - but even throwing out everything above the 90th percentile, the stddev is still at 6188.

They way these are collected as offsets from navigationStart is good, but it seems to me the variation in each of these measurements from that point is cumulative. In other words, domInteractive is more variable because it includes all the potential variations in caching, DNS lookups, response times, etc.

It might be helpful to collect offsets between each timing event and it's predecessor as well. For instance, performance.timing.domInteractive - performance.timing.domLoading would be an interesting handle on blocking CSS or JS issues.

It may be something that could be left to the user as a config option.