A computer-implemented method, comprising: identifying a set of resources in response to crawling a plurality of webpages, each resource in the set of resources being a component of a webpage in the plurality of webpages; for each resource in the set of resources: determining an age of the resource using a timestamp for the resource that indicates a time that a webpage of the plurality of webpages was previously crawled to identify the resource as a component of the webpage; determining a pre-fetch measure of the resource based on the age of the resource and usage information that describes use of the resource by one or more of the plurality of webpages, wherein the pre-fetch measure indicates whether the resource is a candidate for pre-fetching, wherein determining the pre-fetch measure comprises: determining a percentage of webpages from among the plurality of webpages that use the resource to render a digital component at a web browser that loads the webpage; and determining whether the percentage of webpages exceeds a threshold percentage; for each respective resource that is determined to have a pre-fetch measure that exceeds a threshold measure, determining (208, 310) whether an age of the respective resource exceeds a threshold age; for each respective resource that is determined to have an age that exceeds the threshold age and a pre-fetch measure that exceeds a threshold measure, generating an index entry (210, 312) in a pre-fetch index, the index entry including a command to pre-fetch the respective resource based on the determination that the age of the respective resource exceeds the threshold age, wherein at least one index entry is generated at the pre-fetch index, and using the command included in the index entry at the pre-fetch index to pre-fetch a first resource when a client device submits a request to a webpage that uses the first resource that is determined to have an age that exceeds the threshold age. identifying a set of resources in response to crawling a plurality of webpages, each resource in the set of resources being a component of a webpage in the plurality of webpages; for each resource in the set of resources: determining an age of the resource using a timestamp for the resource that indicates a time that a webpage of the plurality of webpages was previously crawled to identify the resource as a component of the webpage; determining a pre-fetch measure of the resource based on the age of the resource and usage information that describes use of the resource by one or more of the plurality of webpages, wherein the pre-fetch measure indicates whether the resource is a candidate for pre-fetching, determining an age of the resource using a timestamp for the resource that indicates a time that a webpage of the plurality of webpages was previously crawled to identify the resource as a component of the webpage; determining a pre-fetch measure of the resource based on the age of the resource and usage information that describes use of the resource by one or more of the plurality of webpages, wherein the pre-fetch measure indicates whether the resource is a candidate for pre-fetching, wherein determining the pre-fetch measure comprises: determining a percentage of webpages from among the plurality of webpages that use the resource to render a digital component at a web browser that loads the webpage; and determining whether the percentage of webpages exceeds a threshold percentage; for each respective resource that is determined to have a pre-fetch measure that exceeds a threshold measure, determining (208, 310) whether an age of the respective resource exceeds a threshold age; determining a percentage of webpages from among the plurality of webpages that use the resource to render a digital component at a web browser that loads the webpage; and determining whether the percentage of webpages exceeds a threshold percentage; for each respective resource that is determined to have a pre-fetch measure that exceeds a threshold measure, determining (208, 310) whether an age of the respective resource exceeds a threshold age; for each respective resource that is determined to have an age that exceeds the threshold age and a pre-fetch measure that exceeds a threshold measure, generating an index entry (210, 312) in a pre-fetch index, the index entry including a command to pre-fetch the respective resource based on the determination that the age of the respective resource exceeds the threshold age, wherein at least one index entry is generated at the pre-fetch index, and using the command included in the index entry at the pre-fetch index to pre-fetch a first resource when a client device submits a request to a webpage that uses the first resource that is determined to have an age that exceeds the threshold age.