Performance Recording is done on a dashboard to optimize and speed up the dashboard. It is there so you can find how long each of the objects are taking to run.
Here are the steps you would take to record performance on your Tableau Dashboard.
Open the view for which you want to record performance.
When you open a view, Tableau Server appends ":iid=<n>" after the URL.
This is a session ID. For example:
Type :record_performance=yes& at the end of the view URL, immediately before the session ID. For example:
Click the Refresh button in the toolbar.
Click here to find out other Tableau Server Web Commands
You are also able to this on Tableau Desktop. Check out this guide to see how to find out what takes how long to load