---
title: Performance and timers
description: Measure timing, performance, and timing of subrequests and other operations.
image: https://developers.cloudflare.com/dev-products-preview.png
---

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# Performance and timers

## Background

The Workers runtime supports a subset of the [Performance API ↗](https://developer.mozilla.org/en-US/docs/Web/API/Performance), used to measure timing and performance, as well as timing of subrequests and other operations.

### `performance.now()`

The [performance.now() method ↗](https://developer.mozilla.org/en-US/docs/Web/API/Performance/now) returns timestamp in milliseconds, representing the time elapsed since `performance.timeOrigin`.

When Workers are deployed to Cloudflare, as a security measure to [mitigate against Spectre attacks](https://developers.cloudflare.com/workers/reference/security-model/#step-1-disallow-timers-and-multi-threading), APIs that return timers, including [performance.now() ↗](https://developer.mozilla.org/en-US/docs/Web/API/Performance/now) and [Date.now() ↗](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global%5FObjects/Date/now), only advance or increment after I/O occurs. Consider the following examples:

Time is frozen — start will have the exact same value as end.

```

const start = performance.now();

for (let i = 0; i < 1e6; i++) {

  // do expensive work

}

const end = performance.now();

const timing = end - start; // 0


```

Time advances, because a subrequest has occurred between start and end.

```

const start = performance.now();

const response = await fetch("https://developers.cloudflare.com/");

const end = performance.now();

const timing = end - start; // duration of the subrequest to developers.cloudflare.com


```

By wrapping a subrequest in calls to `performance.now()` or `Date.now()` APIs, you can measure the timing of a subrequest, fetching a key from KV, an object from R2, or any other form of I/O in your Worker.

In local development, however, timers will increment regardless of whether I/O happens or not. This means that if you need to measure timing of a piece of code that is CPU intensive, that does not involve I/O, you can run your Worker locally, via [Wrangler](https://developers.cloudflare.com/workers/wrangler/), which uses the open-source Workers runtime, [workerd ↗](https://github.com/cloudflare/workerd) — the same runtime that your Worker runs in when deployed to Cloudflare.

### `performance.timeOrigin`

The [performance.timeOrigin ↗](https://developer.mozilla.org/en-US/docs/Web/API/Performance/timeOrigin) API is a read-only property that returns a baseline timestamp to base other measurements off of.

In the Workers runtime, the `timeOrigin` property returns 0.

```json
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```