Optimize with Chrome’s Layers Panel

Unlocking the Secrets of Slow Websites: A Guide to Reflows and Repaints

Are you tired of scratching your head, wondering why your website is slow despite having the latest tech stack and optimized load times? The answer might lie in the way browsers render pages. In this article, we’ll delve into the world of reflows and repaints, and explore how they can impact your website’s performance.

The Browser’s Rendering Process

To understand reflows and repaints, let’s first take a look at how browsers render pages:

1. Layout: After the DOM is created and styles are recalculated, the browser determines the size and position of each visible HTML node.
2. Paint: The browser rasterizes the vector boxes from the layout phase, creating pixel-perfect representations of each element.
3. Compositing: The painted elements are layered together and displayed on the screen.

Reflows and Repaints: The Culprits Behind Slow Websites

Reflows occur when changes are made to the page that force the browser to recalculate positions or geometry of elements, triggering the layout, paint, and compositing phases. Examples include changing display properties, appending elements, or animating element sizes.

Repaints happen when changes affect only paint properties, such as altering background colors or box shadows. Unlike reflows, repaints only impact elements on the same layer as the changed node.

Optimizing Reflows and Repaints

To minimize the impact of reflows and repaints:

• Use the will-change property or translate3D hack to promote elements to their own layer, reducing repaints.
• Scope changes to smaller document parts using layers.
• Avoid overusing layers, as each consumes device memory.
• Animate opacity and transform properties, which are added at the compositing stage.

Debugging Reflows and Repaints

Identifying reflows and repaints can be challenging. Fortunately, Chrome Dev Tools offers the Layers panel, which visualizes layers, repaints, and reflows. To access it:

1. Open Chrome Dev Tools.
2. Click on the three dots in the top-right corner.
3. Select “More Tools” > “Layers”.

Real-Life Example: Optimizing a Rating Widget

Using the Layers panel, we discovered that a rating widget was causing unnecessary repaints due to its stacking context. By adjusting the z-index property, we eliminated the repaints and improved performance.

Tips for Using the Layers Panel

• Be aware of the panel’s performance impact and potential crashes.
• Use the Rendering tab as a fallback if the Layers panel is too heavy.
• Combine the Layers panel with recordings from the Performance tab for comprehensive debugging.

By understanding reflows and repaints, you can unlock the secrets of slow websites and optimize your pages for better performance. Remember to test thoroughly and use the tools available to you to create a seamless user experience.