On the previous page, I gave an overview of the logs; here I shall show you which logs to use, and what to look for to help you tune your workbooks.

I've previously explained that short of using the built-in functions such as Groups, Sets, Parameters, Aliases and Textboxes, all calculations are reinterpreted by the Tableau VizQL engine to the native language of your source; though despite the conversion, this isn't always the most optimum of queries. By using the information captured to the logs, we can see the query (eg SQL) that VizQL generated, copy the query to our data studio such as SQL Server Management Studio, and then re0execute the same code with Show Plan to see the effect of the query. 

Data source optimisation really is limited to a database such as Microsoft SQL Server, Teradata, Oracle etc; though there might still be opportunities to speed-up flat-file too.

Remember, your data source(s) determine where Tableau logs the query information to, though as a minimum, you will need:

Given their size, both in row count, but also row length, working with the logs is soo much more easier when opened in a source code editor rather than the Windows Notepad, just be sure that your editor can open and refresh actively in-use files.
Though there are plenty of free and proprietary source-code editors available, my favourite is Notepad++ which you can get from here, based on its size, minimal resource consumption, great feature list, language detection and support, and that it can be ran from a portable exe, so no install necessary