It could be a number of things; you have a lot of rows / columns in your dataset, you are creating joins / relationships between your sources and it's resulting in a complex calculation, you have a number of table calculations etc etc.
Unfortunately your description alone is not enough to provide you with any real actionable insight. Can you upload a Tableau Packaged Workbook so we can take a look?
I extracted it to a tde and used that as a data source. Problem solved.