What are the possible reasons for slow performance in Tableau?

Usually one of a small number of causes, and worth diagnosing with a performance recording before changing anything.

• A slow live connection. The workbook can only be as fast as the query. Switching to an extract is often the single biggest improvement. - Too much data. Pulling row-level detail when the view shows monthly totals. Filter at source, hide unused fields, and aggregate the extract. - Too many marks. A view with hundreds of thousands of marks is slow to render and unreadable anyway. - Too many sheets on a dashboard. Every sheet is at least one query. - Expensive filters. Quick filters set to Only Relevant Values run an extra query each; condition and top-N filters compute over the full set first. - Costly calculations. Long nested IF chains, string operations, and multiple nested LOD expressions each add work. - Complex joins or blends, particularly blends across different sources, which mean several round trips.

The order to work in is: record first, fix the biggest contributor, record again. Optimising by intuition usually targets rendering when the problem is the query.