Top Core Web Vitals Recommendations for 2023

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The Google Core Web Vitals team understand the amount of web performance recommendations is overwhelming and many don't know where to start. We've been working on identifying the 9 key recommendations (3 per Core Web Vital), which we think will have the most impact and which we recommend sites look at first. This talk will explain what they are, and why they are our top 2023 recommendations.

This talk has been presented at JSNation 2023, check out the latest edition of this JavaScript Conference.

FAQ

Core Web Vitals are three new metrics developed by Google to measure user experience on websites. These metrics include Largest Contentful Paint (LCP), Cumulative Layout Shift (CLS), and First Input Delay (FID). They are designed to provide a standardized way of measuring web performance across different websites.

Largest Contentful Paint (LCP) measures the time from when a user clicks on a link to when the largest piece of content on the page, usually a banner image or H1 tag, is fully rendered. It is an important metric for assessing loading performance.

Cumulative Layout Shift (CLS) measures the visual stability of a webpage. It quantifies how much the page layout shifts unexpectedly during the loading phase, which can be caused by ads, images, or other content loading asynchronously. A lower CLS score indicates a more stable and user-friendly page.

First Input Delay (FID) measures the time it takes for a page to respond to the first user interaction, such as clicking a button or a link. It is a metric that assesses interactivity and responsiveness. However, FID is set to be replaced by Interaction to Next Paint (INP) in the near future.

Interaction to Next Paint (INP) is a new metric that will replace First Input Delay (FID). INP measures the full interaction experience from the time a user initiates an action until the next paint of the screen, providing a more comprehensive assessment of the web page's responsiveness.

To improve Largest Contentful Paint (LCP), ensure that LCP resources are discoverable in the HTML, prioritize LCP resources using the fetchpriority attribute, and use Content Delivery Networks (CDNs) to optimize document and resource Time to First Byte (TTFB).

To improve Cumulative Layout Shift (CLS), reserve space for images and ads by setting width and height attributes, use min-height for dynamic content areas, ensure your site is eligible for the back forward cache (BF cache), and avoid using layout-inducing CSS properties for animations.

To improve First Input Delay (FID), avoid or break up long tasks in JavaScript, reduce the amount of unnecessary JavaScript, and avoid large rendering updates. Using techniques like concurrent rendering, lazy loading, and optimizing your JavaScript can help make your site more responsive.

Several tools can be used to measure web performance, including Lighthouse, Yellow Lab Tools, WebPageTest, and Chrome DevTools Performance Panel. These tools provide various audits, checks, and detailed information to help developers identify and fix performance issues.

Web performance is crucial because slow websites can negatively impact user experience, leading to higher bounce rates, lower conversions, and reduced user satisfaction. Improving web performance can enhance user engagement, increase site traffic, and ultimately drive better business outcomes.

Barry Pollard
Barry Pollard
29 min
01 Jun, 2023

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Video Summary and Transcription

Google has introduced Core Web Vitals, three new metrics for measuring user experience on websites. They have also provided recommended limits for each metric and announced a new metric called IMP. The talk focuses on web performance recommendations, including optimizing HTML parsing, using the fetch priority API, and optimizing CLS. It also covers optimizing JavaScript performance, avoiding unnecessary third-party content, and optimizing rendering and DOM. These recommendations aim to improve web performance and user experience.

1. Introduction to Core Web Vitals

Short description:

Hello, everybody. I'm Barry. Slow websites suck. There's a lot of web performance advice out there. First of all, you've got to figure out what you've got to measure. We think we kind of solved this. We, being Google, came up with three new three-letter acronyms, the Core Web Vitals. These are three new metrics that Chrome came up with as a way of measuring user experience for their websites.

Hello, everybody. I'm Barry. That was a great introduction, so I will skip past on that and get started straight into the talk.

Slow websites suck. How many people like slow websites? Weirdos. And like, there's a lot of web performance advice out there. Maybe too much. I know because I write a lot of it.

First of all, you've got to figure out what you've got to measure. We love our three-letter acronyms in web performance. There's loads of them, tons of them, and we're just adding more and more continually. Timed first byte, by the way, is a three-letter acronym. The second T doesn't really count for anything, two, who cares? This is kind of overwhelming for particular people who aren't web perf nerds like myself.

We think we kind of solved this. We, being Google, came up with three new three-letter acronyms, the Core Web Vitals. Hands up, who's heard of the Core Web Vitals? Mixed crowd here. Okay. So, these are three new metrics that Chrome came up with as a way of measuring user experience for their websites. And these are ways that we can measure every single website. So, your only website might have your own metrics that you want to use. You might want to look at conversions, you might want to look at bounce rates, you might want to look at signups and that sort of thing. These are more measuring across the board that any website can use.

There's three of them. The Largest Contentful Paint, or LCP, measures the time from when you click on a link to the largest bit of content that's on the page. Typically that's a banner image. Maybe your H1 tag or something like that. Cumulative Layout Shift is my favorite one. It's whenever you go to a site and you start reading and an ad pops in and the thing moves down, and it moves across, and you have no idea and you lose your place and it's really, really irritating. Traditionally we never really measured that before so it's really interesting to have that. And FID, or First Input Delay, is supposed to be the responsiveness metric.

2. Improving Web Performance

Short description:

So when you click on a menu and it doesn't open, and you click again, and then it suddenly registers both and opens and closes really quickly and it's really annoying. And as well as coming up with the metrics, we came up with recommended limits for each of them. If you're under 2.5 seconds for LCP, we say you're good. If you're above 4 seconds, we say you're poor. And anywhere in between is mm, okay. We've just announced that FID is going to be replaced very soon with IMP, a new metric that particularly affects JavaScript people. So we now know what to measure. We've given you nice little things that we think that you should measure there. The question then is how do you use that to improve web performance? We want to answer this question. We want to give a simpler, smaller list and say these are the things you should look at first. We want to have a particular focus on recommendations that we believe are the largest real-world impact. We want to look at recommendations that are relevant and applicable to most websites.

So when you click on a menu and it doesn't open, and you click again, and then it suddenly registers both and opens and closes really quickly and it's really annoying. So we measure that. And as well as coming up with the metrics, we came up with recommended limits for each of them. If you're under 2.5 seconds for LCP, we say you're good. If you're above 4 seconds, we say you're poor. And anywhere in between is mm, okay.

One thing to note is that we've just announced that FID is going to be replaced very soon with IMP, a new metric that we'll talk a good bit about later because that particularly affects JavaScript people and I heard there might be some in the room at the moment. So we'll come back to that one.

Okay. So we now know what to measure. We've given you nice little things that we think that you should measure there. The question then is how do you use that to improve web performance? So we've lots of tools, you can stick it in Lighthouse, it will run 53 performance audits and come back and say these are the things you can do. Yellow Lab Tool is another great tool, it will give you 38 little checks and give you a green tick or a red cross and say look at these things. Web Page test, for anyone who's done any waterfall analysis, it's fantastic. It's 16 pages of stats. And Chrome Dev Tools Performance Panel, if any of you have looked into that is, let's just say there's a lot of detailed information there and apologies to some of the Dev Tools team that I see over there.

So, we're back to the same thing, it's kind of overwhelming again. So we want to answer this question. So, we came up with, I spent a lot of time last year looking at this question. What are the most important recommendations that we can give to developers to help them improve the performance for their users? So, rather than stick it in the Lighthouse telling you, these are 53 things that you could improve but will it actually move the metric or not? We want to give a simpler, smaller list and say these are the things you should look at first. Particularly if you're new to web performance, you haven't really looked at it first, look at these things first and then come back to look at the rest. We want to have a particular focus on recommendations that we believe are the largest real-world impact. So, we're going to sit there and tell you to do this and you're going to spend a lot of time implementing it and it's going to see if not point, not, not, not, one second off your website. You're going to be annoyed and go, okay, yes, maybe technically that's best practice to do this but it took me six months and it didn't really do anything. Thanks very much. So, we're looking at things here that we really think will have an impact. We want to look at recommendations that are relevant and applicable to most websites. So, it's going to be lots of talks here at this conference on React or solid JS or whatever. It's very specific to those or if you're at another conference, on WordPress or whatever. So, we're looking at more general things here that every website should be able to consider and have a look at.

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