A request for content is received at a content server via a first network connection from a client device. The content includes a set of portions represented within a root document. Based upon a download constraint of at least one portion of the content and a network characteristic of the first network connection, at least one portion of the content is deferred for download over a network connection other than the first network connection. The content is sent with each deferred portion of the content replaced with a content stub within the root document. Each content stub identifies the respective portion of the content as a deferred portion of the content and includes a content identifier of the respective deferred portion of the content.