Ahmed, please take a look at the following:
The basic takeaways are:
1) Your SSAS cubes need to be very carefully designed to get the best performance out of Tableau. That may mean that what is best for SSAS/SSRS is NOT what is best for Tableau.
2) Tableau has to translate everything it does with a cube file from VizQL/SQL to MDX - and that takes a lot of resources.
3) Check that your server is sufficiently provisioned for RAM and CPU cores.
4) Review the architecture of your workbook(s) to make certain that you are doing things in Tableau that should be done in Tableau, and move anything else back to the source SSAS cube. This is a trial-and-error process, but the above links should point you at general "good practices" when working with SSAS.