## TL;DR

The Performance API (`performance.now()`, `performance.getEntries()`) provides high-resolution timing data (microsecond precision) for measuring web application performance. It includes the Navigation Timing, Resource Timing, and User Timing APIs.

## Core Explanation

`performance.now()` returns a DOMHighResTimeStamp relative to `timeOrigin` with 5μs precision. `performance.mark('name')` and `measure()` for custom metrics. `performance.getEntriesByType('resource')` lists all loaded resources with timing breakdowns (DNS, TCP, request, response). Core Web Vitals (LCP, INP, CLS) are exposed via PerformanceObserver.

## Further Reading

- [Performance Timeline (W3C)](https://www.w3.org/TR/performance-timeline/)