There's a lot of information out there about visual best practices! A great place to start for Tableau questions in general is the Tableau Reference Guide, created by Data+Science. If you look under Training Videos, you'll see one titled Dashboard Design and Formatting. Just searching "Dashboard" on the reference guide to get a ton of resources. I also really like the resources found on The Data School! Again, search "Dashboards" or "Formatting" to find a bunch of really great resources.
For performance tuning, we use a relatively inexpensive tool from InterWorks called Power Tools for Tableau Desktop. I am still getting familiar with it, but it has great features to help you audit data sources, set up visual best practices, and performance analyzer to help you clean up your workbooks before (or after) publishing them.
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Tableau Reference Guide
The Data School
Power Tools for Tableau