Have you tried to run a performance recording on your workbook. There are many reasons why the workbook could be running slow:
- Many complex calculations
- Heavy usage of data blending
- Very large data-set
- Server specification not able to handle the request
- Queries generated by Tableau taking too long to execute
What is the configuration/specs of your Tableau Server?
To run a performance recording: Create a Performance Recording
This might help you narrow down the cause.