We were dealing with this issue a year and a half ago and it was an absolute pain. We had numerous conversations with the official Microsoft support at the time and, in all honesty, they couldnt help much except for advising us that the issue was our SQL Server (queriestaking more than 5 seconds...); limit the number of the visuals in a single report. In a nutshell, limit the amount of data to be fetched; make sure to put solid indexing.
1. Check the SQL query performance. End-result in Power BI depends on the performance of your backend.
2. Further tune it with indexing.
3. Consider having an SSAS solution of your data instead of taping on schema tables directly.