What are the possible reasons for slow performance in Tableau?

The usual suspects, roughly in order of how often they turn out to be the cause:

• Querying a slow live source where an extract would do. - Bringing back more data than the view needs — row-level detail behind a monthly summary. - Too many marks in a single view. - Too many sheets on one dashboard, each generating its own query. - Quick filters set to Only Relevant Values, each costing an extra query. - Nested or repeated calculations, especially string operations and stacked LOD expressions. - Blends across sources, which mean multiple round trips rather than one query. - Custom SQL that the optimiser cannot simplify.

The method matters more than the list: take a performance recording, fix whatever dominates it, and record again. Most wasted effort in tuning comes from optimising the part that was already fast.