When creating a dashpart report, you have the option to Always Rebuild the report. If you select this check box, it means that every time a dashboard is loaded, the report query will get re-run.
For best performance, leave the Always Rebuild check box cleared. If this option is cleared, the dashboard will look to see if there are currently any archived reports for the given dashpart and display that report when the dashboard is loaded.
This means that you can schedule reports to run in off hours using the process server scheduler. When the dashboard is opened in the morning, the data will be ready for the report.