The cache management will help you improve the performance while the user interacts with the report document in the ReportViewer, as pages and their resources will be cached and the whole document would not be reprocessed on navigating, exporting or printing. This settings are applied in the web.config file as explained in the Configuring Cache help article.
If you are exporting the report programmatically, the whole report, including its resources are hold in memory and processed at once. Thus our suggestion is to filter the data using report parameters; or if the machine has enough resources to handle the report document generation - increase the timeouts of the service at both client and server sides.
Keep in mind the native Silverlight printing also requires more resources and disabling it may reduce the used memory (set UseNativePrinting to false
Additional details how to improve the performance are available in the Performance Considerations
If you need further help, please open a support ticket and elaborate on the scenario, the amount of data, the report complexity, when the CPU and memory usage peaks(is it specific rendering format). We will be waiting as well the requested project, including test data and results from profiling at your end.
Check out the Telerik Platform - the only platform that combines a rich set of UI tools with powerful cloud services to develop web, hybrid and native mobile apps.