1. A method comprising: receiving, by a computing system, a request from a client device for a target structured document; in response to the received request from the client device and in a first response phase: accessing, by the computing system, a data structure comprising an entry for the target structured document and one or more first resources associated with the target structured document; for each of one or more of the first resources associated with the target structured document: computing a probability for the first resource that represents a likelihood that the first resource will be included in a response to a future request for the target structured document; comparing the probability to a first predetermined threshold; and when the probability exceeds the first predetermined threshold, identifying the first resource as a selected first resource for the target structured document; generating, by the computing system, a first response portion comprising one or more of: one or more of the selected first resources; or one or more references to one or more of the selected first resources; sending, by the computing system, the first response portion to the client device; and further in response to the received request from the client device and in a second response phase taking place after sending the first response portion: for each of one or more second resources associated with the target structured document: computing a probability for the second resource that represents a likelihood that the second resource will be included in a response to a future request for the target structured document; comparing the probability to a second predetermined threshold; and when the probability exceeds the second predetermined threshold, identifying the second resource as a selected second resource for the target structured document; generating, by the computing system, a second response portion comprising structured document language code for the target structured document and one or more of: one or more of the selected second resources associated with the target structured document; or one or more references to one or more of the selected second resources; and sending, by the computing system, the second response portion to the same client device.
3. The method of claim 1 , wherein none of the selected second resources are identical to any of the selected first resources.