Calculate FCP SPA Metrics

How to Calculate FCP SPA

Understanding FCP

First Contentful Paint (FCP) measures the time from when the user first navigates to a page to when any part of the page's content is rendered on the screen. This metric is crucial for assessing perceived load speed and enhancing user experience.

Tools Required for FCP Calculation

To measure FCP accurately, utilize tools like Lighthouse and Chrome DevTools for lab environments. For field measurements, PageSpeed Insights, the Chrome User Experience Report, and the Search Console (Speed Report) are recommended.

Implementing Measurement in JavaScript

The Paint Timing API and the web-vitals JavaScript library are essential for measuring FCP in JavaScript. Activate these tools using PerformanceObserver to capture paint timings, including those from iframes.

Key Measurement Approaches

FCP should be measured from navigational start time. For prerendered pages, start from activationStart. Ensure to account for pages loaded in background tabs or restored from the back/forward cache.

Interpreting FCP Results

An FCP score of 1.8 seconds or less is considered good, while a score exceeding 3.0 seconds may indicate performance issues. Attaining an optimal FCP score significantly uplifts the user's experience on your web application.

How to Calculate FCP SPA

First Contentful Paint (FCP) is a critical performance metric that indicates the time at which the first piece of content appears on a webpage. FCP is crucial for assessing perceived load speed and overall user experience. This guide explains how to effectively measure FCP for Single Page Applications (SPA).

Using the Paint Timing API

The Paint Timing API is integral for pinpointing FCP events. Implement it by utilizing the PerformanceObserver to listen for 'paint' entries, specifically the 'first-contentful-paint'. These entries can then be logged to the console for analysis, ensuring that you capture the earliest point at which the user sees visible content on the screen.

Utilizing the web-vitals JavaScript Library

For a more streamlined approach, the web-vitals.js library can be used. This library simplifies the process by providing a function onFCP, which automatically handles the complexity of measuring FCP, especially in challenging scenarios like iframes or SPA routes. This method is ideal for ensuring accurate measurement across varied content types, from text to images.

Enhancing Measurement for SPA

Since Single Page Applications often do not perform hard navigations, enabling soft navigations becomes essential. From Chrome 110, soft navigations can be activated using the --enable-experimental-web-platform-features command line argument. This setting allows the capturing of FCP during SPA route changes, using techniques such as server-side rendering (SSR) or static site generations (SSG) to simulate initial page loading.

Calculating Optimal FCP Times

For optimal user experience, aim for an FCP time of 1.8 seconds or less. Measurements can be conducted both in laboratory environments using tools like Lighthouse and Chrome DevTools, or in the field with PageSpeed Insights and the Chrome User Experience Report. This dual approach ensures both controlled and real-world conditions are considered, helping developers optimize FCP effectively.

By following these methodologies, developers can enhance the performance of their Single Page Applications, ensuring minimal delay in delivering content to users, thus improving overall site engagement and user satisfaction.

Examples of Calculating FCP SPA

The following examples illustrate different scenarios for calculating the First Contentful Paint (FCP) Single Page Application (SPA) metric. FCP measures the time from when a page starts loading to when any part of the page's content is rendered on the screen. For SPAs, this metric is crucial for understanding initial page load performance.

Example 1: Basic Static SPA

In a static SPA where HTML is loaded once, calculate FCP by marking the time when the first text or image is rendered. If the initial content loads at 1.2 seconds, then FCP for this SPA is 1.2 seconds.

Example 2: SPA with Dynamic Content

For an SPA that fetches content dynamically, initiate an FCP calculation when the main JavaScript bundle is loaded. Suppose content rendering completes at 2.5 seconds from the start time; the FCP would be 2.5 seconds.

Example 3: SPA with Third-Party Scripts

Including third-party scripts might delay the FCP. If the third-party scripts finish loading by 0.8 seconds and the main content renders by 1.5 seconds, record the FCP at 1.5 seconds, indicating an area for optimization.

Example 4: React SPA on Fast Network

A React SPA on a fast network might load content quicker. If the Document Object Model (DOM) is ready and content is painted at 0.9 seconds, FCP would be 0.9 seconds, showcasing efficient performance.

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