Comparing The Performance Of CSV And Excel Data Sources In Power Query

My posts from two weeks ago (see here and here) on using Process Monitor to troubleshoot the performance of Power Query queries made me wonder about another question: how does the performance of reading data from CSV files compare to the performance of reading data from Excel files? I think most experienced Power Query users in either Power BI or Excel know that Excel data sources perform pretty badly but I had never done any proper tests.

Bron : Chris Webb’s BI Blog
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Troubleshooting Data Refresh Performance Issues With OData Data Sources In Power BI And Excel Using Fiddler

A lot of people have problems with the performance of OData data sources when loading data into Power BI and Excel. One possible cause of these problems is query folding not taking place – if this is the case then the Power Query engine will be requesting more data that is needed and applying any filters itself locally, rather than requesting filtered data from the data source. How do you know whether query folding is taking place or not though?

Bron : Chris Webb’s BI Blog
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