mPulse Feature Release - LCP Metrics
By Nic Jansma on
Table of Contents
The mPulse team has added two new measurements that track different aspects of Largest Contentful Paint (LCP):
- LCP Transfer Time (timer)
- LCP Decoded Body Size (metric)
These new timers and metrics give mPulse customers better insight into how their LCP elements and images are behaving in the real world (their speed and size), and can be found in both the Legacy and New UI:
Using DSWB widgets you can easily see how many bytes were fetched, the transfer time, and the percentage of the screen that was covered by the object for different screen sizes:
Read below for how both of these new measurements are calculated!
LCP Transfer Time
LCP Transfer Time measures the network duration of the resource (file) that triggered the LCP, such as the image or video.
This timer is a companion to the Largest Contentful Paint timer, but it only focuses on the duration of the image triggering the LCP, rather than all of the other phases of the navigation that LCP is affected by.
This metric may be less "noisy" than measuring LCP itself, as it excludes all of the network phases that were required to get to LCP. Customers may want to set alerts on LCP Transfer Time to ensure their primary LCP resources are being loaded quickly.
LCP Decoded Body Size
LCP Decoded Body Size measures the size of the resource (file) that triggered the LCP, such as the image or video.
Note that "... Decoded ..." denotes that this is the size of the file after decoding has been applied (which is often the result of gz or br decompression), if necessary. Most images are not compressed, so this will often be roughly the same as the resource's Transfer Size (minus the headers length).
Using this metric, you can easily measure the weight of LCP resources, and how it may vary by dimension. For example, this chart compares LCP vs. the time and size of the LCP resource, by Page Group:
As expected, larger resources often take longer to download.
Customers may want to set alerts on LCP Decoded Body Size to ensure their image budget doesn't exceed their expectations.
Caveats
- Both of these metrics will only have data if the Largest Contentful Paint was triggered by a downloadable resource (e.g. image, video). LCP may be triggered by a text block or other non-resource content, which would result in these metrics being empty.
- LCP Decoded Body Size requires the resource to be either same-origin or have the
Timing-Allow-Originheader applied so the size information can be retrieved from ResourceTiming
Summary
These metrics are available for use in all of the standard Dashboards and Widgets in the mPulse UI.
Please let us know if you have any feedback!