Synth vs BRUM Metrics: Setting Expectations
Ideally we should not compare BRUM metrics and Synth metrics directly as their use cases and data representation differ, so comparing them doesn't set the right expectations. Though many of the metrics collected by Browser Synthetic are identical to those collected by Browser RUM, they are not exactly the same and they are not meant to serve the same purpose.
For example, in case of Synth, the Visually Complete Time tells you what time your page was completely loaded. It doesn't consider the EURT metrics or other BRUM metrics for this specifically. The same concept applies to calculating resource timing. In Synth, we show a separate Data Downloaded Before Fully Loaded metric, which is intended to show any resources loaded after the page load event.
Defining Synth vs. BRUM Load Time Metrics:
Visually Complete Time - The time when your first viewport looks visually complete (when the page looks visually complete in the first viewport). It is usually less than the load event or EURT time for a page.
EURT - Indicates when page loading is complete.
- EURT - Load event time for base pages
- EURT ! - Load event time for virtual pages
However, sometimes the first viewport can change even after load time depending on how the website is built. Based on that, the Visually Complete Time can be less or more than the load time. It is when the website in the first viewport looks visually full.
To compare EURT and the visual load time of a page, you can club the EURT metric from browserRecords table with synth_session_records table as demonstrated below:
SELECT browserRecords.pagename, browserRecords.metrics.`End User Response Time (ms)` FROM synth_session_records WHERE browserRecords.pageurl = "/abc"
Restrictions
1. We do not currently show Visually Complete Time as a metric using ADQL; we already have an enhancement for this though.
2. Some visual metrics (like Visually Complete time, Speed Index) may not always be present. There are currently two known issues regarding visual metrics:
- In some cases, Chrome sessions may not display visual metrics because of a bug. This typically happens on pages with JS redirects, but may also happen elsewhere. This is a Chrome-specific problem and we are working on a solution.
- A Windows issue will sometimes prevent these metrics from being computed for all browsers. When this happens, the agent needs to be rebooted. This is infrequent because we receive alerts and react as quickly as possible.
