It occurred to me that I should make sure we mean the same things by "refresh" and "interval."
By "interval," I mean a 30-minute block of time. Any time you see "interval" in the PureCloud interface, it *should* mean 30 minute block of time.
When I say "refresh" I mean "the view updates to take into account new information." So for Performance > Overview, that view will be refreshing in close to real-time in order to incorporate new calls, etc. into its overall calculations for the current 30-minute interval.
As far as I know, there is not a workaround to look at 15 minute intervals instead of 30-minute intervals. However, I'd recommend looking at Performance > Queues Activity and Performance > Queues Activity > Queue. That last one, in particular, lets you view and compare things like ASA, service level, etc. for different blocks of time. For example, you can see how the Support queue did from 12:00 to 12:30 versus 12:30-1:30, or you can look at data for a whole day or week (https://help.mypurecloud.com/articles/queues-performance-views/).
As for creating your own dashboard, that depends what you're looking to do. In the web/browser app, you can choose to view by 30 minute interval, day, week, or custom date range in the Queues Performance view, and some performance views allow you to select different statistics. The iOS supervisor app (https://help.mypurecloud.com/articles/download-mobile-apps-2/) gives you a little more flexibility in creating dashboards, in that you can choose which metrics to view for each agent/queue, but once again, you can't change the interval.