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A site's performance is poor if visitors perceive it is poor. To measure that perception, all the tools listed in Recommended diagnostic tools use the following (each tool may have other metrics as well):

• First Contentful Paint—Time required to paint the first text or image.
• Speed Index—Time required to visibly populate a page's content.
• Time to Interactive—Time required until a page is interactive; this includes the time until all initially visible links respond to clicks or taps.
• Total Blocking Time—Sum of all time periods between First Contentful Paint and Time to Interactive, when task length exceeds 50 milliseconds, expressed in ms.
• Largest Contentful Paint—Time required to paint the largest text or image.