A computer system comprising: at least one processor; and memory storing instructions that, when executed by the at least one processor, cause the system to: identify a URL pattern common to a plurality of embedded URLs stored in fetch records; select a sample of URLs from the plurality of embedded URLs; for each URL in the sample, determine whether the URL is an optional resource by: generating a first rendering result for an embedder of the URL using content for the URL; generating a second rendering result for the embedder without using the content for the URL, calculating a similarity score for the URL by comparing the first rendering result to the second rendering result; determining that the URL is an optional resource when the similarity score is greater than a similarity threshold; and responsive to determining a predetermined quantity of the URLs in the sample are optional resources, store the URL pattern in a data store of optional resource patterns; wherein responsive to a request to render a web page comprising an embedded resource having a URL that matches a URL pattern in the data store of optional resources, the web page is rendered as if the embedded resource is unavailable without fetching content for the embedded resource. at least one processor; and memory storing instructions that, when executed by the at least one processor, cause the system to: identify a URL pattern common to a plurality of embedded URLs stored in fetch records; select a sample of URLs from the plurality of embedded URLs; for each URL in the sample, determine whether the URL is an optional resource by: generating a first rendering result for an embedder of the URL using content for the URL; generating a second rendering result for the embedder without using the content for the URL, calculating a similarity score for the URL by comparing the first rendering result to the second rendering result; determining that the URL is an optional resource when the similarity score is greater than a similarity threshold; and responsive to determining a predetermined quantity of the URLs in the sample are optional resources, store the URL pattern in a data store of optional resource patterns; wherein responsive to a request to render a web page comprising an embedded resource having a URL that matches a URL pattern in the data store of optional resources, the web page is rendered as if the embedded resource is unavailable without fetching content for the embedded resource. identify a URL pattern common to a plurality of embedded URLs stored in fetch records; select a sample of URLs from the plurality of embedded URLs; for each URL in the sample, determine whether the URL is an optional resource by: generating a first rendering result for an embedder of the URL using content for the URL; generating a second rendering result for the embedder without using the content for the URL, calculating a similarity score for the URL by comparing the first rendering result to the second rendering result; determining that the URL is an optional resource when the similarity score is greater than a similarity threshold; and generating a first rendering result for an embedder of the URL using content for the URL; generating a second rendering result for the embedder without using the content for the URL, calculating a similarity score for the URL by comparing the first rendering result to the second rendering result; determining that the URL is an optional resource when the similarity score is greater than a similarity threshold; and responsive to determining a predetermined quantity of the URLs in the sample are optional resources, store the URL pattern in a data store of optional resource patterns; wherein responsive to a request to render a web page comprising an embedded resource having a URL that matches a URL pattern in the data store of optional resources, the web page is rendered as if the embedded resource is unavailable without fetching content for the embedded resource.