1. Premium Workspace - If your workspace is premium capacity workspace then you need to understand the traffice. The same premium capacity can be assigned to multiple workspaces. Check with Premium capacity admin and monitor the Premium Metrics to understand the memory and CPU usage on your schedule time.
2. Database - Check the traffice in your database side. If you are considering any RDBMS source then check with DB admin and understand any other schedules are going on and what is the allocation for your DB account to run the query. Is there any change in the DB configuration etc..
3. Volumne of your data - Check the size of your dataset and make sure to apply the incremental refersh to get the high refresh speed. Also ensure query folding is happening.