Each case type eventually have one starter flow. So, you can go to your starter flow (Pega 7 and above it's pyStartCase if you leverage Case Designer) rule definition. Go to Process Tab and use Start Conditions and/or Security options. If you use security option, only users who have at least one of the privileges listed to use this flow as shown in my attachment for this section.
Hope it will help you to achieve your requirement.
As far as my knowledge goes, The Dashboard contains multiple charts whose source would be report definitions. The performance would be the performance of report definitions. You can get a better performance if you can fine tune any custom charts that you have in the dashboard. One common thing would be to remove any unwanted columns from report definition and having only the optimised properties in the report definition.
On a side note, I would recommend you to create a new post for a new query as it would have better visibility and more chances of responses.