When you design a report, you have the option to Execute it using F5 but you may also have the option to Render it using F6.

Rendering is enabled if you have previously executed your report, and if you did not change the model definitions.
Note that you should keep the Report Viewer window open...

In this case you can just render the report without querying again the database, the models previously built are used again.
This can save a lot of time when you design a report and you want just modify the presentation (actually the View properties).

If you have several views in your report, you have the corresponding Execute/Render options for the current view (F10/F11)

Conclusion: At design time, just use Render when possible !