The aspects include browser systems and methods of loading/rendering a webpage by preprocessing scripts within the web document (HTML page) in a sandboxed script engine to discover resources not explicitly requested in the web document so such resources can be pre-fetched and downloaded speculatively. The sandboxed execution of scripts and downloading of discovered resources may proceed in parallel with the performance of other browser operations (e.g., HTML parsing) and other resource requests. The sandboxed script engine may be isolated or separated from the other browser components. The sandboxed script engine may operate to speedup the process of identifying resources inconsistent with standard script execution processes.