Read each metrics section#

The Performance tab is a stack of sections. Each section answers one question about the session. The sections read from top to bottom, from the broad shape of the capture down to the detail of individual connections. This page explains what each one tells you.

Page Vitals and Error Summary#

Page Vitals is the headline. It shows the request count, the time to first byte, the wall-clock duration of the session, and the total transferred size and total resource size of everything the session pulled down. Where the capture recorded them, it also shows the DOMContentLoaded and load events. Use it to judge whether a session was heavy or light before you look at anything else.

Error Summary counts what went wrong: 4xx client errors, 5xx server errors, and requests that failed outright, with a total across all three. It also breaks the count down by status code. It lists the failing requests, so you can select one and see it in the inspector.

Hotspots#

Hotspots shows the requests to look at first. It holds three lists:

• Slowest Requests: ranked by time.
• Largest Responses: ranked by bytes transferred.
• Redirect Chains: requests that bounced through others before they landed.

Select an entry to open the request behind it. When you chase a slow or heavy session, start here.

Caching and compression#

Caching & Compression shows how much of the session came from cache and how well responses were compressed. It reports cache hits, cache misses, and the hit rate, plus the bytes saved by compression. It also lists the largest responses that were not cacheable, so you can see where repeat visits fetch the same content again.

Protocol and connections#

Protocol Breakdown groups the session by HTTP version and, where it is known, TLS version. It also reports the number of unique connections and the average requests carried per connection. That average tells you how well the session reused connections instead of a new connection for each request.

First-party and third-party weight#

First-Party vs Third-Party splits the session by domain against the root site. It counts first-party and third-party requests and the bytes each pulled down, and it ranks the third-party domains by weight. Use it to see how much of a page's cost came from the site itself, and how much from everything else it loaded.

Resource Timing Percentiles#

Resource Timing Percentiles breaks request time down by connection phase, from DNS lookup and connection setup through to the wait for the first byte and the receive. It reports percentiles rather than a single average. You can therefore tell whether a phase was slow across the board or slow only for a few requests.