An client-side performance optimization system for streamed applications provides several approaches for fulfilling client-side application code and data file requests for streamed applications. A streaming file system or file driver is installed on the client system that receives and fulfills application code and data requests from a persistent cache or the streaming application server. The client or the server can initiate the prefetching of application code and data to improve interactive application performance. A client-to-client communication mechanism allows local application customization to travel from one client machine to another without involving server communication. Applications are patched or upgraded via a change in the root directory for that application. The client can be notified of application upgrades by the server which can be marked as mandatory, in which case the client will force the application to be upgraded. The server broadcasts an application program's code and data and any client that is interested in that particular application program stores the broadcasted code and data for later use.