Power BI Desktop - Resource Usage - CPU/RAM/Disk priorities

If you are using Direct Query, then the processing moves from your PowerBI client directly to the SQL Server, so getting a faster computer won't help you out.

The fact that PowerBI works well when connected to the Power BI Service (and... I assume when you import the data?), the problem is not your local machine, but the SQL Server you are connecting to.

Here's the guidance on using Direct Query, https://docs.microsoft.com/en-us/power-bi/connect-data/desktop-directquery-about but to speed things up, you need to have a faster SQL Server, or have the IT department/DBA's look for opportunities to optimize the SQL server, or display fewer visualizations in your dashboards, or... you might consider talking to your stakeholders to determine how important the "semi-live" requirement is. 

If you are importing the data and refreshing in the service, you can setup refreshes to take place 8 times a day.  If they are okay with having data that is generally less than an hour old, in exchange for a much faster report... they might like that trade-off.