When restricted by the browser is Same Origin Policy, the same server that hosts the application must take on the task of fetching the third-party content and sending it to the client. The server acts as a client to the third-party service in addition to its usual server function. Using the server as a proxy for every client transaction means that a large number of users could cause undue server load. Applications using this technique would need to be designed to be scalable on the server side, using multiple coordinated servers to handle the request load.
