Created a new dataflow, and connected to Salesforce (Objects). It took 9.5 minutes to list the set of Salesforce objects. Then upon selecting an object, the preview of the data still hadnt appeared after 5 minutes.
The same operation in the Power BI Desktop took about 2 seconds to show the list of tables, then another 2s to show the preview of the selected table.
In my opinion, if you want to get a solution for this kind of dataflow problem as soon as possible based on your specific situation, I suggest you to open a support ticket to get direct help from the technical support team of Microsoft to have a remote session them to help you to make sure if this is an issue existing in the Power BI.