1. A computer-implemented method comprising: accepting a set of one or more document identifiers; for each of the one or more document identifiers of the set, and using a client device characterized by a first combination of processor characteristics, memory characteristics, operating system characteristics, browser characteristics, client device location, request time of day, request day of week, and request time of year: appending a randomized parameter to a request for a document identified by the document identifier; requesting the document from a first serving facility using the request with the appended randomized parameter, receiving the document from the first serving facility in response to the requesting, and rendering the received document, the document including a first type of ad; determining a first elapsed time from an earliest time of requesting, using the client device characterized by the first combination, a first document identified by the set of one or more document identifiers to a latest time of completion of the rendering, using the client device characterized by the first combination, of a last document identified by the set of one or more document identifiers; for each of the one or more document identifiers of the set, and using a client device characterized by a second combination of processor characteristics, memory characteristics, operating system characteristics, browser characteristics, client device location, request time of day, request day of week, and request time of year: appending a randomized parameter to a request for a document identified by the document identifier; requesting the document from the first serving facility using the request with the appended randomized parameter, receiving the document from the first serving facility in response to the requesting, and rendering the received document, the document including the first type of ad; determining a second elapsed time from an earliest time of requesting, using the client device characterized by the second combination, the first document identified by the set of one or more document identifiers to a latest time of completion of the rendering, using the client device characterized by the second combination, of the last document identified by the set of one or more document identifiers; and determining whether to send subsequent requests for documents having the same content as the document identified by the document identifier and including a different, second type of ad to a different, second serving facility based at least in part on a comparison of the first elapsed time to the second elapsed time and the first and second types of ads; wherein the determining is performed using one or more processors.