Dataset scheduled Refresh is taking too long
Community Supporthi
First, power bi scheduled refresh delay is a normal phenomenon in power bi.
Having configured a refresh schedule, the dataset settings page informs you about the next refresh time, as in the screenshot above. If you want to refresh the data sooner, such as to test your gateway and data source configuration, perform an on-demand refresh by using the Refresh Now option in the dataset menu in the nav pane. On-demand refreshes don't affect the next scheduled refresh time.
Note also that the configured refresh time might not be the exact time when Power BI starts the next scheduled process. Power BI starts scheduled refreshes on a best effort basis. The target is to initiate the refresh within 15 minutes of the scheduled time slot, but a delay of up to one hour can occur if the service can't allocate the required resources sooner.
https://docs.microsoft.com/en-us/power-bi/connect-data/refresh-data#setting-a-refresh-schedule
and for why scheduled Refresh is slower than demand refresh, have a look this post:
I think the issue with scheduled refresh is that it does not start at the top or bottom of the hour, even though that's the time you have chosen. For example, you schedule a daily refresh for 12:00 am. It goes into a queue at 12:00, but it can take 15 + minutes to start. However, when you do a manual refresh, it starts immediately. This is a limitation of shared capacity model.
Regards,
Lin