1. A method performed by data processing apparatus, the method comprising: for each website of a first plurality of websites, each website being a collection of resources served from a domain: obtaining, by a data processing apparatus, duration measurements of user device session visits to resources of the website for a plurality of different users, each duration measurement being a duration of time that elapses between a user device requesting the resource and the user device requesting another resource, and the user device session visits being a sequence of visits to resources of the website without an intervening visit to a resource of another website; determining a website duration score that is proportional to duration measurements for the website; determining, for each category to which second plurality of website belong, the second plurality of websites being a subset of the first plurality of websites, a cross-website category duration score for the category based on the website duration scores of the second plurality of websites, wherein the cross-website category duration score is a measure of duration scores of other websites that belong to the category; for each website of a first plurality of websites: obtaining, by the data processing apparatus, data describing a plurality of categories to which the website belongs; determining, by the data processing apparatus, for only each category of the plurality of categories to which the website belongs, a category duration score for the category based on the website duration score for the website and the cross-website category duration score for the category; and determining, by the data processing apparatus, from one or more of the category duration scores, a duration performance score for the website, the duration performance score proportional to the one or more category duration scores from which the duration performance score is determined.