Power BI Continuous Improvement cycle invariably has an element that looks at improving the performance of your queries. DAX can become slow especially when you start to add more elements to what a data model is doing, over time you will find parts of it are slower, using Performance Analyser and DAX studio makes it possible for you to look at what is going on to make a visual appear, but when you look deeper into what is going on especially as you get into the cross highlighting and filtering the more time it will take for a page to load, so understanding the variables and the way it will interact is key. Remember though speed isn’t always the most important thing, but it is a detractor for certain audiences.
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https://daxstudio.org/