1. A computer system that acts as an intermediary between user devices and content sites, the computer system comprising one or more computing devices, and being programmed to at least: retrieve a content page from a content site; generate a preview of the content page comprising: an image of at least a portion of the content page; and markup code comprising a first instruction to transmit a request for the image and a second instruction to transmit a follow-up request for the content page; and in response to a request from a user device for the content page: transmit the markup code to the user device upon determining that the request is not a follow-up request; and transmit a non-preview version of the content page to the user device upon determining that the request is a follow-up request.