What you need to know
A page can look fast on a developer's machine but feel slow on a phone or unreliable network. Field metrics help reveal delays and visual shifts people experience in normal use.
How to apply it
Measure representative routes, identify the biggest visible image and long tasks, reserve layout space, and retest after each change. Use both lab tests for debugging and field data when available.
Common mistake to avoid
Do not optimize a score while making the page harder to use. Real content and interaction quality matter alongside a metric.