Timeline panel

The Timeline panel  allows you to record detailed information
about what happens when a web page loads and then monitor it in
real time as events trigger actions.
The Timeline panel is where you can really see the different parts of a web
browser engine doing its thing. The Timeline panel gives you a complete rundown
on what it takes to load a web page along with how long each part took
and what the sequence of events was.

To get started with the Timeline panel:
1. Click the Record button at the bottom of the panel.
This is the round button to the left of the trashcan icon. The button
turns red when you click it to show that it’s recording.
2. Open a web page you’ve never visited before — www.weirdal.com,
for instance.
You see a burst of activity in the Timeline panel as the various parts of
the page are requested, loaded, parsed, laid out, and painted.
3. When the page is done loading, click the Record button again to stop
recording.
The Record button turns back to black.