Computing devices can be used to request content from other computing devices over a communication network. In a common application, a client computing device can request a web page from a server computing device via the internet. Browser application software on the client computing device typically retrieves a requested web page, processes resource identifiers embedded in the web page to generate requests for additional resources (e.g., images, script files, etc.), and renders the content for presentation. From the perspective of a user of a client computing device, a user experience can be defined in terms of the performance and latencies associated with obtaining and rendering the requested network content on the client computing device. Latencies and performance limitations of any of the above processes may diminish the user experience.
Optimizations and other improvements may be implemented to reduce latency and otherwise improve the user experience. For example, some content items may be cached at a client device, and future requests for the content items can be fulfilled from the local cache. As another example, proxy servers may be used to cache content for multiple clients and then pass the content to the clients that originally requested the content. Subsequent requests to the proxy server can be served from the proxy cache if the content is present, rather than requiring retrieval of the content from an origin server. As a further example, content delivery networks (“CDNs”) can locate points-of-presence closer (in either a geographic or networking context) to certain client devices. Content requests may be made to the CDN rather than the origin server, and the CDN can fulfill the request more quickly due to being closer to the requesting device. Some proxy servers and other intermediary systems can pre-render or otherwise pre-process content prior to responding to a client request. Such pre-processing enables some of the processing burden to be offloaded from the client device to the intermediary system.