 Philip Walton [X](https://twitter.com/philwalton) [GitHub](https://github.com/philipwalton) [LinkedIn](https://www.linkedin.com/in/waltonphilip) [Mastodon](https://toot.cafe/@philipwalton) [Bluesky](https://bsky.app/profile/philipwalton.com) [Homepage](https://philipwalton.com/)
There's a lot of value in having [user-centric metrics](https://web.dev/articles/user-centric-performance-metrics.md.txt) that you can measure, universally, on any given website. These metrics let you:
- Understand how real users experience the web as a whole.
- Compare your site to a competitor's.
- Track useful and actionable data in your analytics tools without needing to write custom code.
Universal metrics offer a good baseline, but in many cases you need to measure *more* than just these metrics in order to capture the full experience for your particular site.
Custom metrics let you measure aspects of your site's experience that may only apply to your site, such as:
- How long it takes for a single page app (SPA) to transition from one "page" to another.
- How long it takes for a page to display data fetched from a database for logged-in users.
- How long it takes for a server-side-rendered (SSR) app to [hydrate](https://addyosmani.com/blog/rehydration/).
- The cache hit rate for resources loaded by returning visitors.
- The event latency of click or keyboard events in a game.
## APIs to measure custom metrics
Historically web developers haven't had many low-level APIs to measure performance, and as a result they've had to resort to hacks in order to measure whether a site was performing well.
For example, it's possible to determine whether the main thread is blocked due to long-running JavaScript tasks by running a `requestAnimationFrame` loop and calculating the delta between each frame. If the delta is significantly longer than the display's framerate, you can report that as a long task. Such hacks are not recommended, though, because they actually affect performance themselves (by draining battery, for example).
The first rule of effective performance measurement is to make sure your performance measurement techniques aren't causing performance issues themselves. So for any custom metrics you measure on your site, it's best to use one of the following APIs if possible.
[](https://web.dev/newsletter?nl_cta=true)
### Performance Observer API
Browser Support
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[Source](https://developer.mozilla.org/docs/Web/API/PerformanceObserver)
The Performance Observer API is the mechanism that collects and displays data from all other performance APIs discussed on this page. Understanding it is critical to getting good data.
You can use `PerformanceObserver` to passively subscribe to performance-related events. This lets API callbacks fire during [idle periods](https://w3c.github.io/requestidlecallback/#idle-periods), which means they usually won't interfere with page performance.
To create a `PerformanceObserver`, pass it a callback to be run whenever new performance entries are dispatched. Then you tell the observer what types of entries to listen for using the [`observe()`](https://developer.mozilla.org/docs/Web/API/PerformanceObserver/observe) method:
```javascript
const po = new PerformanceObserver((list) => {
for (const entry of list.getEntries()) {
// Log the entry and all associated details.
console.log(entry.toJSON());
}
});
po.observe({type: 'some-entry-type'});
```
The following sections list all the various entry types available for observing, but in newer browsers, you can inspect what entry types are available through the static [`PerformanceObserver.supportedEntryTypes`](https://w3c.github.io/performance-timeline/#supportedentrytypes-attribute) property.
> [!NOTE]
> **Note:** The object passed to the `observe()` method can also specify an `entryTypes` array (in order to observe more than one entry type using the same observer). While specifying `entryTypes` is an older option with wider browser support, using `type` is now preferred, as it lets you specify additional entry-specific observation configuration (such as the `buffered` flag, discussed next).
#### Observe entries that already happened
By default, `PerformanceObserver` objects can only observe entries as they occur. This can cause problems if you want to lazy-load your performance analytics code so it doesn't block higher-priority resources.
To get historical entries (after they've occurred), set the `buffered` flag to `true` when you call `observe()`. The browser will include historical entries from its [performance entry buffer](https://w3c.github.io/performance-timeline/#dfn-performance-entry-buffer) the first time that your `PerformanceObserver` callback is called, up to the [maximum buffer size for that type](https://w3c.github.io/timing-entrytypes-registry/#registry).
```javascript
po.observe({
type: 'some-entry-type',
buffered: true,
});
```
> [!NOTE]
> **Note:** To avoid memory issues, the performance entry buffer is not unlimited. For most typical page loads it's unlikely that the buffer will fill up and entries will be missed.
#### Legacy performance APIs to avoid
Prior to the Performance Observer API, developers could access performance entries using the following three methods defined on the [`performance`](https://w3c.github.io/performance-timeline/) object:
- [`getEntries()`](https://developer.mozilla.org/docs/Web/API/Performance/getEntries)
- [`getEntriesByName()`](https://developer.mozilla.org/docs/Web/API/Performance/getEntriesByName)
- [`getEntriesByType()`](https://developer.mozilla.org/docs/Web/API/Performance/getEntriesByType)
While these APIs are still supported, their usage is not recommended because they don't let you listen for when new entries are emitted. In addition, many new APIs (such as `largest-contentful-paint`) are not exposed through the `performance` object, they're only exposed through `PerformanceObserver`.
Unless you specifically need compatibility with Internet Explorer, it's best to avoid these methods in your code and use `PerformanceObserver` going forward.
### User Timing API
Browser Support
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[Source](https://developer.mozilla.org/docs/Web/API/PerformanceMark)
The [User Timing API](https://w3c.github.io/user-timing/) is a general-purpose
measurement API for time-based metrics. It lets you arbitrarily mark points in
time and then measure the duration between those marks later.
```javascript
// Record the time immediately before running a task.
performance.mark('myTask:start');
await doMyTask();
// Record the time immediately after running a task.
performance.mark('myTask:end');
// Measure the delta between the start and end of the task
performance.measure('myTask', 'myTask:start', 'myTask:end');
```
While APIs like `Date.now()` or `performance.now()` give you similar abilities, the benefit of using the User Timing API is it integrates well with performance tooling. For example, Chrome DevTools visualizes [User Timing measurements in the Performance panel](https://developer.chrome.com/blog/new-in-devtools-67/#tabs), and many analytics providers will also automatically track any measurements you make and send the duration data to their analytics backend.
To report User Timing measurements, you can use [PerformanceObserver](https://web.dev/articles/custom-metrics.md.txt#performance-observer) and register to observe entries of type `measure`:
```javascript
// Create the performance observer.
const po = new PerformanceObserver((list) => {
for (const entry of list.getEntries()) {
// Log the entry and all associated details.
console.log(entry.toJSON());
}
});
// Start listening for `measure` entries to be dispatched.
po.observe({type: 'measure', buffered: true});
```
### Long Tasks API
Browser Support
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[Source](https://developer.mozilla.org/docs/Web/API/PerformanceLongTaskTiming)
The [Long Tasks API](https://w3c.github.io/longtasks/) is useful for knowing when the browser's main thread is blocked for long enough to affect frame rate or input latency. The API will report any tasks that execute for longer than 50 milliseconds.
> [!CAUTION]
> **Caution:** The Long Tasks API has been superseded by the newer Long Animation Frames API (discussed next). Consider whether to use that API instead.
Anytime you need to run expensive code, or load and execute large scripts, it's useful to track whether that code blocks the main thread. In fact, many higher-level metrics are built on top of the Long Tasks API themselves (such as [Time to Interactive (TTI)](https://web.dev/articles/tti.md.txt) and [Total Blocking Time (TBT)](https://developer.chrome.com/docs/lighthouse/performance/lighthouse-total-blocking-time)).
To determine when long tasks happen, you can use [PerformanceObserver](https://developer.mozilla.org/docs/Web/API/PerformanceObserver) and register to observe entries of type `longtask`:
```javascript
// Create the performance observer.
const po = new PerformanceObserver((list) => {
for (const entry of list.getEntries()) {
// Log the entry and all associated details.
console.log(entry.toJSON());
}
});
// Start listening for `longtask` entries to be dispatched.
po.observe({type: 'longtask', buffered: true});
```
### Long Animation Frames API
Browser Support
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[Source](https://developer.mozilla.org/docs/Web/API/PerformanceLongAnimationFrameTiming)
The [Long Animation Frames API](https://w3c.github.io/long-animation-frames/) is a new iteration of the Long Tasks API that looks at *long frames* ---rather than *long tasks* ---of over 50 milliseconds. This addresses some [shortcomings of the Long Tasks API](https://developer.chrome.com/docs/web-platform/long-animation-frames#long-tasks-shortcomings), including better attribution and the a wider scope of potentially problematic delays.
To determine when long frames happen, you can use [PerformanceObserver](https://developer.mozilla.org/docs/Web/API/PerformanceObserver) and register to observe entries of type `long-animation-frame`:
```javascript
// Create the performance observer.
const po = new PerformanceObserver((list) => {
for (const entry of list.getEntries()) {
// Log the entry and all associated details.
console.log(entry.toJSON());
}
});
// Start listening for `long-animation-frame` entries to be dispatched.
po.observe({type: 'long-animation-frame', buffered: true});
```
### Element Timing API
Browser Support
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[Source](https://developer.mozilla.org/docs/Web/API/PerformanceElementTiming)
The [Largest Contentful Paint (LCP)](https://web.dev/articles/lcp.md.txt) metric is useful for knowing when the largest image or text block was painted to the screen, but in some cases you want to measure the render time of a different element.
For these cases, use the [Element Timing API](https://wicg.github.io/element-timing/). The LCP API is actually built on top of the Element Timing API and adds automatic reporting of the largest contentful element, but you can also report on other elements by explicitly adding the `elementtiming` attribute to them, and registering a PerformanceObserver to observe the `element` entry type.
```html
![]()
This is text I care about.
```
> [!TIP]
> **Tip:** The types of elements considered for Largest Contentful Paint are the same as those observable using the Element Timing API. If you add the `elementtiming` attribute to an element that isn't one of those types, the attribute will be ignored.
### Event Timing API
Browser Support
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stop-color='%23fff44f' stop-opacity='.2'/%3E%3Cstop offset='.6' stop-color='%23fff44f' stop-opacity='0'/%3E%3C/linearGradient%3E%3C/defs%3E%3Cpath d='M479 166c-11-25-32-52-49-60a249 249 0 0 1 25 73c-27-68-73-95-111-155a255 255 0 0 1-8-14 44 44 0 0 1-4-9 1 1 0 0 0 0-1 1 1 0 0 0-1 0c-60 35-81 101-83 134a120 120 0 0 0-66 25 71 71 0 0 0-6-5 111 111 0 0 1-1-58c-25 11-44 29-58 44-9-12-9-52-8-60l-8 4a175 175 0 0 0-24 21 210 210 0 0 0-22 26 203 203 0 0 0-32 73l-1 2-2 15a229 229 0 0 0-4 34v1a240 240 0 0 0 477 40l1-9c5-41 0-84-15-121zM202 355l3 1-3-1zm55-145zm198-31z' fill='url(%23ff-a)'/%3E%3Cpath d='M479 166c-11-25-32-52-49-60 14 26 22 53 25 72v1a207 207 0 0 1-206 279c-113-3-212-87-231-197-3-17 0-26 2-40-2 11-3 14-4 34v1a240 240 0 0 0 477 40l1-9c5-41 0-84-15-121z' fill='url(%23ff-b)'/%3E%3Cpath d='M479 166c-11-25-32-52-49-60 14 26 22 53 25 72v1a207 207 0 0 1-206 279c-113-3-212-87-231-197-3-17 0-26 2-40-2 11-3 14-4 34v1a240 240 0 0 0 477 40l1-9c5-41 0-84-15-121z' fill='url(%23ff-c)'/%3E%3Cpath 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- ' xlink:href='%23s-b'%3E%3Cstop offset='0' stop-color='%2324a5f3' stop-opacity='0' /%3E%3Cstop offset='1' stop-color='%231e8ceb' /%3E%3C/radialGradient%3E%3CradialGradient id='s-j' cx='109.3' cy='13.8' r='93.1' gradientTransform='matrix(-.02 1.1 -1.04 -.02 137 -115)' xlink:href='%23s-b'%3E%3Cstop offset='0' stop-opacity='0' /%3E%3Cstop offset='1' stop-color='%235488d6' stop-opacity='0' /%3E%3Cstop offset='1' stop-color='%235d96eb' /%3E%3C/radialGradient%3E%3C/defs%3E%3Crect width='220' height='220' x='22' y='-107' fill='url(%23s-a)' ry='49' transform='matrix(.57 0 0 .57 187 256)' /%3E%3Cg transform='translate(194 190)'%3E%3Ccircle cx='67.8' cy='67.7' fill='url(%23s-c)' paint-order='stroke fill markers' r='54' /%3E%3Ccircle cx='-69.9' cy='69.3' fill='url(%23s-i)' transform='translate(138 -2)' r='54' /%3E%3C/g%3E%3Cellipse cx='120' cy='14.2' fill='url(%23s-j)' rx='93.1' ry='93.7' transform='matrix(.58 0 0 .58 192 250)' /%3E%3Cg transform='matrix(.58 0 0 .57 197 182)'%3E%3Cpath fill='%23cac7c8' d='M46 192h1l72-48-7-9-66 57Z' /%3E%3Cpath fill='%23fbfffc' d='M46 191v1l66-57-7-9-59 65Z' /%3E%3Cpath fill='url(%23s-d)' d='m119 144-7-9 66-57-59 66Z' /%3E%3Cpath fill='%23fb645c' d='m105 126 7 9 66-57-1-1-72 49Z' /%3E%3C/g%3E%3Cpath stroke='%23fff' stroke-linecap='round' stroke-miterlimit='1' stroke-width='1.3' d='m287 278 3-2m-12-17 8-2m-8-3h4m-4-13 8 2m-8 3h4m-1-13 7 3m-4-11 7 4m-2-11 6 6m0-12 6 7m1-11 4 6m4-10 3 7m5-9 2 7m15-7-1 7m10-5-3 7m11-4-4 7m11-2-5 6m16 7-7 4m10 4-7 3m10 6-8 1m8 16-8-2m5 10-7-3m4 11-7-4m2 11-6-5m0 11-5-6m-2 11-4-7m-4 11-3-8m-6 10-1-8m-16 8 2-8m-10 5 3-7m-11 4 4-7m-11 2 5-6m-8 3 3-3m4 8 2-3m5 8 2-4m6 7 1-4m8 5v-4m8 4v-4m9 3-1-4m9 1-2-4m9 0-2-4m9-2-3-3m8-4-3-2m8-5-4-2m7-6-4-1m5-8h-4m4-8h-4m3-9-4 1m1-9-4 2m-1-9-3 2m-2-9-3 3m-4-8-2 3m-5-8-2 4m-6-6-1 3m-8-5v4m-8-4v4m-9-2 1 3m-9 0 2 3m-9 1 2 3m-9 2 3 3m-8 4 3 2m-8 5 4 2m-7 6 4 1m-4 25 4-1m-2 5 7-3m-6 7 4-2m-2 6 7-4m-13-21h8m41-41v-8m0 99v-8m49-42h-8' transform='translate(-65 8)' /%3E%3C/svg%3E)
[Source](https://developer.mozilla.org/docs/Web/API/PerformanceEventTiming)
The [Interaction to Next Paint (INP)](https://web.dev/articles/inp.md.txt) metric assesses overall page responsiveness by observing all click, tap, and keyboard interactions throughout the life of a page. A page's INP is most often the interaction that took the longest to complete, from the time the user initiated the interaction, to the time the browser paints the next frame showing the visual result of the user's input.
The INP metric is made possible by the [Event Timing API](https://wicg.github.io/event-timing/). This API exposes a number of timestamps that occur during the event lifecycle, including:
- [**`startTime`**](https://w3c.github.io/performance-timeline/#dom-performanceentry-starttime): the time when the browser receives the event.
- [**`processingStart`**](https://wicg.github.io/event-timing/#dom-performanceeventtiming-processingstart): the time when the browser is able to begin processing event handlers for the event.
- [**`processingEnd`**](https://wicg.github.io/event-timing/#dom-performanceeventtiming-processingend): time when the browser finishes executing all synchronous code initiated from event handlers for this event.
- [**`duration`**](https://wicg.github.io/event-timing/#dom-performanceeventtiming-processingstart): the time (rounded to 8 milliseconds for security reasons) between when the browser receives the event until it's able to paint the next frame after finishing executing all synchronous code initiated from the event handlers.
The following example shows how to use these these values to create custom measurements:
```javascript
const po = new PerformanceObserver((entryList) => {
// Get the last interaction observed:
const entries = Array.from(entryList.getEntries()).forEach((entry) => {
// Get various bits of interaction data:
const inputDelay = entry.processingStart - entry.startTime;
const processingTime = entry.processingEnd - entry.processingStart;
const presentationDelay = entry.startTime + entry.duration - entry.processingEnd;
const duration = entry.duration;
const eventType = entry.name;
const target = entry.target || "(not set)"
console.log("--- INTERACTION ---");
console.log(`Input delay (ms): ${inputDelay}`);
console.log(`Event handler processing time (ms): ${processingTime}`);
console.log(`Presentation delay (ms): ${presentationDelay}`);
console.log(`Total event duration (ms): ${duration}`);
console.log(`Event type: ${eventType}`);
console.log(target);
});
});
// A durationThreshold of 16ms is necessary to include more
// interactions, since the default is 104ms. The minimum
// durationThreshold is 16ms.
po.observe({type: 'event', buffered: true, durationThreshold: 16});
```
### Resource Timing API
Browser Support
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[Source](https://developer.mozilla.org/docs/Web/API/PerformanceResourceTiming)
The [Resource Timing API](https://w3c.github.io/resource-timing/) gives developers detailed insight into how resources for a particular page were loaded. Despite the name of the API, the information it provides is not just limited to timing data (though there's [plenty of that](https://w3c.github.io/resource-timing/#processing-model)). Other data you can access includes:
- [**`initiatorType`**](https://w3c.github.io/resource-timing/#dom-performanceresourcetiming-initiatortype): how the resource was fetched: such as from a `