Author here. The server timing metrics here are actually extracted from an APM tracing tool (Scout). Tracing services generally do not give immediate feedback

Author here. The server timing metrics here are actually extracted from an APM tracing tool (Scout). Tracing services generally do not give immediate feedback on the timing breakdown of a web request. At worst, the metrics are heavily aggregated. At best, you'll need to wait a couple of minutes for a trace. The Server Timing API (which is how this works) give immediate performance information, shortening the feedback loop and allowing you to do a quick gut-check on a slow request before jumping to your tracing tool.