Performance and timers

Background

The Workers runtime supports a subset of the Performance API, used to measure timing and performance, as well as timing of subrequests and other operations.

performance.now()

The performance.now() method 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, APIs that return timers, including performance.now() and 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, which uses the open-source Workers runtime, workerd — the same runtime that your Worker runs in when deployed to Cloudflare.

performance.timeOrigin

The 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.