1. use Cube rather than database directly
2. Try switching back connection type from extract to live or from live to extract to see if there is any change
3. try to extract first dashboard data source only and keep rest as it is if
4. Use filtered data
5. Use required columns.
6. reduce number of calculations at the tableau end as you already getting aggregated data from cube itself.
7. Look into your server machine configuration as well such as RAM etc.
8. Reduce as many background jobs/service in your server
Let me if any one of the above suggestions could help you out with performance.