13. The system of claim 1 , wherein: the plurality of resource mappings include a first subset of resource mappings, including the particular resource mapping, that are each assigned to the requested resource; and at least two resource mappings within the first subset of resource mappings designate different portions of the executable code of the requested resource as non-essential for rendering a visually complete presentation of the requested resource.
13. The system of claim 1 , wherein: the plurality of resource mappings include a first subset of resource mappings, including the particular resource mapping, that are each assigned to the requested resource; and at least two resource mappings within the first subset of resource mappings designate different portions of the executable code of the requested resource as non-essential for rendering a visually complete presentation of the requested resource. 15. The system of claim 13 , wherein the operations comprise: determining that a user of the client device that submitted the request for the resource belongs to a particular cluster of users of multiple clusters of users; determining that the particular resource mapping is associated with the particular cluster of users to which the user of the client device that submitted the request for the resource belongs; and selecting the particular resource mapping from the first subset of resource mappings based on the particular resource mapping being associated with the particular cluster of users to which the user of the client device that submitted the request for the resource belongs.