Page Speed Metrics You Should Measure

Webspeed Index uses both lab testing and real user field measurements to track page speed and site performance metrics, but with so many metrics relating to page speed to choose from how do you know which ones are important and what to measure first?

In this article we highlight 6 top key metrics that are critical for delivering a great user experience.

1 - Largest Contentful Paint (LCP)

Largest Contentful Paint measures perceived loading speed. This is achieved by measuring how fast the main content of the web page has loaded; a fast LCP reassures the user a page is useful.

To provide a good user experience, LCP should occur within 2.5 seconds of when the request for the web page is made.

Why track it?

Google determines if a web page is providing a high quality user experience, based on its page experience signals.

As LCP is also part of Google"s Core Web Vital metrics, then good LCP measurements will contribute to ranking how the web page ranks in search results.

2 - First Input Delay (FID)

First Input Delay is a measure of interactivity and measures the time from when a user first interacts with your page to the time when the browser is able to respond to that interaction.

To provide a good user experience, pages should have an FID of less than 100 milliseconds.

FID is a field-based metric and may not be available to review, As a lab-testing metric Total Blocking Time (TBT) is considered a suitable surrogate for FID.

TBT is similar to FID as a measure of responsiveness as it highlights how long the browser is blocked from processing and therefore preventing the user to interact with the web page as it is loading.

To provide a good user experience, web pages should have a TBT of under 300 milliseconds.

Why track it?

FID directly affects user experience in relation to onsite conversion and bounce rate.

When a web page is loaded on a smart-phone or desktop, the inability to interact with it results in a poor user experience and may cause annoyance and frustration.

FID is a Core Web Vital, and therefore contributes to how the web page ranks in search results and a high FID may be penalized by Google in its ranking processes.

Improvements in TBT is considered to beneficial to FID as well.

3 - Cumulative Layout Shift (CLS)

Cumulative Layout Shift (CLS) is an important, user-centric metric for measuring visual stability because it helps quantify how often users experience movement (layout shift) of the content in the visible area (viewport) before the web page becomes interactive.

A low CLS improves overall user experience and the Web Vitals recommendation for a good user experience is to maintain a CLS of less than 0.1.

Why track it?

Unexpected movement of components of the web page in the viewport impedes the ability of the user to view and interact with the content. Too much movement of the web page components is considered as a poor user experience and may cause frustration with the web page, website and possibly the brand.

As a Core Web Vital metric if user experiences are poor, this may impact on search rank, purchase confidence, site experience and trust.

4 - Speed Index (SI)

Speed Index is a measure of how quickly the visible part of a web page (viewport) is displayed. Although Speed Index is expressed in seconds it is not a linear measurement of time but is a calculated metric based how much of the viewport is complete at specific time intervals, such as 100ms, and how long it takes to be completed.

This makes Speed Index dependent of many different factors including the size of the viewport (screen size), computing power of the device and the network speed.

A Speed Index value of under 1000 (1s) represents best practice, but a value between 1000 (1s) and 2500 (2.5s) generally delivers a quality user experience.

Why track it?

Speed Index has been used as a measure of user experience as it identifies how quickly the web page is visible.

As it is reliant on many other factors, together with other metrics, such as Time to First Byte (TTFB) and First Contentful Paint (FCP) it can be used to identify potential performance bottlenecks on the web page.

5 - First Contentful Paint (FCP)

First Contentful Paint identifies when the first non-white pixel is painted onto the device screen (viewport).

It is an important metric as it is the first visible sign that the web page is being loaded by the browser. FCP is similar to the Render Start time and generally the terms can be used synonymously.

The Best Practice guideline for FCP is 750ms or faster.

Why track it?

FCP has a direct impact on the Speed Index of the web page, consequently, to lower the Speed Index and improve user experience, FCP should be achieved as quickly as possible.

6 - Time to First Byte (TTFB)

Time to First Byte (TTFB) is the time that it takes from the initial web page (navigation) request, as initiated by the browser, to the browser receiving the first byte of the content.

TTFB is measured in milliseconds and although it measures a linear time event, TTFB is a complex metric that is made up several component timings from different timed activities that occur in delivering the first byte.

Best Practice guidelines for TTFB is under 250ms for a cable-connected network and 500ms on a cellular-mobile connection.

Why track it?

TTFB is a critical metric to track and improve as the browser is unable to proceed with any activity for building the web page until the first resource, typically the HTML of the web page, has been received and processed.

Consequently, the faster TTFB can be delivered, the quicker the browser can begin building the requested web page.

Summary

Keeping in mind what matters most to the customer is key in knowing what page speed elements to track.

Webspeed Index provides visibility and enables you track the performance all of your key metrics over time. Benchmarking against other web pages, both internal and competitive and visualising if a web page is improving or regressing.

Contact Webspeed Index today to get a health check on the key metrics of your pages.