A request is received at a proxy server for a web page, the request originating from a client network application of a client device. The requested web page includes multiple references to multiple images. The proxy server retrieves the requested web page. The proxy server modifies code of the retrieved web page such that the client network application will not, for each one of images, request that image until the location where that image is to be displayed is within a viewport of the client network application or within a defined distance from the viewport of the client network application. The proxy server transmits the modified web page to the client device.