Embodiments optimize web page downloads through a customized manifest to reduce latency and network data consumption. A proxy device creates or selects a pre-defined manifest identifying page resources associated with the web page. The manifest is customized by identifying the page resources already cached on a computing device. A single package of resources representing the web page is created. The single package of resources excludes the identified page resources already cached on the computing device. The computing device renders the web page using the single package and the page resources already cached. Some embodiments perform web crawling to create manifests for other web pages.