Marco Russo explains why opening some Power BI files might take so long:
There could be many reasons for that, but if you have calculated columns and/or calculated tables in your model, you should be aware that they could be the reasons why this happens. It could be, so I want to explain when this happens.
The short explanation is the following: when you open a PBIX file, Power BI Desktop automatically recalculates those calculated columns and calculated tables that depend on a volatile formula.
Read on for the longer explanation, which includes a (possibly incomplete) list of volatile formulas.