Round Trip Time (RTT) is a metric that measures the time it takes for a network request to travel from the user’s browser to the server and back. It is a key factor in determining the latency that users experience.
In Calibre, RTT is collected from Google's Chrome User Experience Report (CrUX) and made available in the CrUX Dashboard and Free Core Web Vitals Test tool. CrUX provides RTT data at the 75th percentile.
RTT is typically measured in milliseconds (ms) and involves capturing the time at which a request is sent and the time at which the response is received. This measurement includes all the delays in the network path between the user and the server.
Round Trip Time collected by Chrome User Experience Report is based on recent browsing latency experienced by visitors to your site.
RTT is not nessarily representative of the latency experienced for direct visits to your site, but by recent browsing latency experienced by visitors that also visit your site.
RTT is often confused with Time to First Byte (TTFB). While both metrics are related to network performance, they measure different aspects of the network request lifecycle. In order to grasp what users experience, it is important to consider both metrics as key indicators of network performance.
You can use RTT as an early indicator of latency that your users experience, while TTFB can be used to understand server performance.
Historically the Network Information API and CrUX dataset has used Effective Connection Type as a mechanism to describe user networking performance using labels such as '4G', '3G', '2G', 'slow-2G' etc.
In Chrome, RTT is also calculated using the Network Information API, but rather than using the Effective Connection Type labels, the actual RTT value is used. This provides a more accurate representation of the network performance experienced by users.
With the announcement of RTT's inclusion to the CrUX dataset, it is expected that ECT will be eventually deprecated in favour of RTT.