Can we use Power to deal with real-time data sources, very large tables, and transactional details? Yes, but it requires some planning and proper design. Out of the box, Power BI is super-fast with moderate data volumes and data schemas optimized for analytic reporting. How big is your data? Millions or billions of rows? Gigabytes or terabytes? - we can handle that.
Moving beyond the basics, learn to use DirectQuery alongside in-memory Import mode in composite models to access very large tables with real-time results; models and reports that combine interactive "dashboard-style" reports and drill-through to transactional details. Take a journey and learn to use the Power BI platform to achieve the best of both worlds: performance and scale.