A deep dive into optimizing LCP

Summary

In this video, Philip Walton discusses optimizing Largest Contentful Paint (LCP), a core web vitals metric. He explains the importance of LCP, provides a framework for improving it, and offers practical steps like prioritizing LCP resource loading, reducing render delays, optimizing resource load times, and minimizing Time to First Byte (TTFB).

Takeaways

• 😀 Largest Contentful Paint (LCP) is a core web vitals metric that measures the time from when a user starts loading a web page until the largest image or text block within the viewport finishes rendering.
• 🏁 Google recommends aiming for an LCP of 2.5 seconds or less for at least 75% of all page visits to provide a good loading experience.
• 📊 The 75th percentile of LCP times is crucial, and it represents the value that is 75% or 3/4 of the way through a sorted list of LCP times.
• 🔍 Improving LCP involves optimizing the experience for enough users so that at least 75% of them are within the good threshold, not just targeting a specific set of users.
• 🌐 LCP is the core web vitals metric that sites struggle with the most, with only 52.7% of sites meeting the good LCP threshold.
• 🔍 Optimizing LCP involves breaking it down into smaller, more manageable problems: Time To First Byte (TTFB), LCP resource load delay, LCP resource load time, and element render delay.
• 🚀 The key to improving LCP is to identify bottlenecks in the loading and rendering process and address them, such as reducing TTFB, optimizing resource load times, and ensuring elements render quickly.
• 🛠️ General best practices for optimizing LCP include prioritizing the loading of the LCP resource, reducing render-blocking resources, optimizing image and font resources, and using CDNs for faster delivery.
• 🌐 Real-world data from HTTP Archive suggests that resource load delay might be the biggest bottleneck for LCP, indicating a need for better prioritization and loading strategies.
• 📈 A step-by-step approach to optimizing LCP includes eliminating unnecessary resource load delay, ensuring the LCP element can render as soon as its resource finishes loading, reducing the load time of the LCP resource, and improving Time To First Byte.

Q & A

What does LCP stand for and why is it important?

LCP stands for Largest Contentful Paint. It is important because it measures the time from when a user starts loading a web page until the largest image or text block within the viewport finishes rendering. Optimizing LCP is crucial for providing a good user experience and is one of the three core web vitals metrics recommended by Google.

What is the recommended LCP score and how does it relate to the user experience?

Google recommends that developers aim for an LCP of 2.5 seconds or less for at least 75% of all page visits. This means that if 75% of the time your pages can render the largest image or text block within 2.5 seconds, then those pages are classified as providing a good loading experience.

How is the 75th percentile of LCP determined and why is it significant?

The 75th percentile of LCP is determined by taking the value that is 75% or 3/4 of the way through a sorted list of LCP times from fastest to slowest. It is significant because it represents the LCP time for the majority of users, indicating the overall performance of the page load.

What happens if performance optimizations only improve the already fast LCP experiences?

If performance optimizations only make the already fast LCP experiences faster, the 75th percentile does not change. To improve the LCP scores at the 75th percentile, the experience for enough users must be improved so that at least 75% of them are within the good threshold.

Why do some developers struggle with optimizing LCP?

Developers struggle with optimizing LCP because there are many factors to consider when optimizing load performance. Often, the optimizations they try do not work or do not help much, making it difficult to identify what will actually make a difference for their specific site.

What are the four main subparts of LCP and how do they contribute to the total LCP time?

The four main subparts of LCP are: 1) Time To First Byte (TTFB), which is the time from when the user initiates page loading until the browser receives the first byte of the HTML document response. 2) LCP resource load delay, which is the time between TTFB and when the browser starts loading the resource needed for LCP. 3) The time it takes to load the LCP resource itself. 4) The element render delay, which is the time from when the LCP resource finishes loading until it's rendered on the screen. These subparts collectively add up to the full LCP time.

Why is it recommended to focus on optimizing the LCP resource load delay?

Optimizing the LCP resource load delay is recommended because it ensures that the LCP resource is prioritized and starts loading as early as possible after the HTML document is received. This can significantly impact the overall LCP time and user experience.

What are some general best practices for reducing the resource load time of the LCP element?

General best practices for reducing the resource load time include optimizing image and web font files, setting proper caching headers, using a CDN to serve resources closer to users, and potentially using server-side rendering or pre-rendering pages as static files.

How can developers use the 80-20 principle in optimizing LCP?

The 80-20 principle suggests that about 80% of the time should be spent making network requests needed to render the LCP element, and 20% of the time should be allocated to everything else. This principle helps in identifying opportunities to improve LCP by focusing on the most impactful optimizations first.

What is the role of server-side rendering in optimizing LCP?

Server-side rendering plays a crucial role in optimizing LCP by allowing the HTML that's delivered to already contain the markup when the browser receives it. This means the browser doesn't have to wait for the JavaScript to finish loading before it can render the images, reducing the element render delay.