When you’re looking at the Log Analytics data for your Power BI Premium dataset, or studying a Profiler trace, you may see a command called ExtAuth in the OperationDetailName column:
It is documented here as “An internal service-generated command to perform authentication” – and as such it’s not something you have any control over. It may be triggered when a report is opened or a dataset refresh is started.
However you may wonder why it’s so slow sometimes. This is because datasets in the Power BI Service are not held in memory 100% of the time (they are sometimes paged out to disk) although they do need to be in memory before they can be queried or refreshed. When a dataset is not held in memory but the Power BI Service needs to connect to it, the dataset is loaded into the Analysis Services engine and into memory and this can take some time, especially if the dataset is large. The ExtAuth event is often the event that triggers the load of a dataset into memory and therefore the time required to load the dataset into memory is associated with it.
[Thanks to Akshai Mirchandani, Kay Unkroth and Rui Romano for the information in this post]