How Do You Measure The Performance Of Images On A Website?

Story Based Question

You’ve been tasked with improving the performance of an online store that sells clothing. The store has beautiful, high-resolution images of the products, but you’ve noticed that the site is running slower than expected, especially on mobile devices. You want to measure how the images are affecting the site’s overall performance so you can make informed improvements. How do you measure the performance of images on the website?

Exact Answer

To measure the performance of images on a website, you can use tools like Google PageSpeed Insights, Lighthouse, or WebPageTest to analyze page load times, image sizes, and their impact on performance. These tools provide detailed reports on image optimization, including issues like uncompressed images, large file sizes, or missing formats like WebP, and offer suggestions to improve performance.

Explanation

Measuring the performance of images is a crucial step in understanding how they impact page load times and user experience. Slow-loading images can significantly affect the performance of your website, especially for mobile users with slower internet connections. Here’s how to measure image performance:

1. Google PageSpeed Insights: This is one of the most widely used tools for analyzing web performance. It provides a score for your website based on various factors, including image optimization. It will highlight any images that are too large or not in the optimal format, such as WebP. PageSpeed Insights also provides suggestions on how to fix these issues (e.g., reducing image file sizes, serving images in next-gen formats).
2. Lighthouse: Lighthouse, available through Chrome DevTools, is a more in-depth tool that provides a full audit of your website, including image performance. It evaluates page load times and image rendering, offering specific advice on improving image optimization for faster performance. Lighthouse can also suggest using image formats like WebP, lazy loading, and reducing image sizes for better speed.
3. WebPageTest: This tool allows you to test how your website loads in real-world scenarios, simulating different devices and network speeds. WebPageTest provides a detailed waterfall chart that shows when and how images are loading, helping you pinpoint bottlenecks caused by large or unoptimized images.
4. Image-Specific Metrics: Tools like PageSpeed Insights and Lighthouse measure specific aspects of images, such as:
   • Image file size: Large images slow down page load times.
   • Image format: Using outdated formats like JPEG or PNG instead of modern formats like WebP can hurt performance.
   • Responsive images: If images aren’t being served in different sizes for different screen resolutions, they may be unnecessarily large for some users.
   • Lazy loading: If images are not lazy-loaded, they may be blocking the initial page load.
5. Real User Monitoring (RUM): For a more ongoing and live analysis, you can use RUM tools like Google Analytics or New Relic to track user interactions and page load times. This provides real-world data on how your images are affecting the user experience.

Example

You’re analyzing the performance of the clothing store’s homepage. It features several high-quality images of t-shirts, shoes, and accessories. Here’s how you might measure and analyze the image performance:

1. Google PageSpeed Insights Report:
   • You run the homepage through PageSpeed Insights and find that the images are taking up a significant portion of the total page load time.
   • The report shows that some product images are 2MB in size, which is too large for web use.
   • The tool recommends compressing the images, serving them in WebP format, and enabling lazy loading to speed up the initial load.
2. Using Lighthouse:
   • You run Lighthouse and get an in-depth audit, which identifies that the homepage images aren’t responsive. The images are 1500px wide, but on mobile devices, they only take up 300px of space.
   • Lighthouse suggests using the srcset attribute for responsive image delivery, which would allow smaller images to be served on mobile devices, improving performance.
3. WebPageTest Waterfall Chart:
   • WebPageTest shows that several product images are blocking the page from rendering because they’re loading at the same time as other resources, like CSS and JavaScript.
   • The chart indicates that the images are not being lazy-loaded, causing delays in page rendering.
4. Fixing the Issues:
   • Based on the findings from these tools, you compress the product images, serve them in WebP format, and implement lazy loading.
   • You also adjust the images to be responsive using srcset, so mobile users only download smaller images.

After implementing these changes, you re-test the page using these tools and find that the page load time has improved by 30%, and mobile users now experience faster loading times with optimized images.

By using tools like Google PageSpeed Insights, Lighthouse, and WebPageTest, you can accurately measure how your images are affecting website performance and make data-driven decisions to improve loading times, optimize file sizes, and enhance the user experience.