1. A system comprising: one or more computers; and a computer-readable medium coupled to the one or more computers having instructions stored thereon which, when executed by the one or more computers, cause the one or more computers to perform operations comprising: receiving a request for a resource from a client device, wherein the requested resource includes markup language code and executable code that is capable of being executed by an application at the client device; selecting, in response to receiving the request for the resource and from among a plurality of resource mappings that are assigned to respective resources from a plurality of resources, a particular resource mapping that is assigned to the requested resource and that designates: (i) a first portion of the executable code of the requested resource as non-essential to an ability of the application at the client device to render a visually complete presentation of the requested resource, and (ii) a second portion of the executable code of the requested resource as essential to the ability of the application at the client device to render the visually complete presentation of the requested resource; and generating, based on identifying that the particular resource mapping designates the first portion of the executable code as non-essential to the ability of the application at the client device to render the visually complete presentation of the requested resource, a modified version of the requested resource by delimiting the first portion of the executable code or a modified version of the first portion of the executable code, to an exclusion of the second portion of the executable code, within the markup language code of the requested resource in a manner that prevents the first portion of the executable code or the modified version of the first portion of the executable code from being executed when the application at the client device initially loads the modified version of the requested resource; and transmitting, to the client device, the modified version of the requested resource.