Experienced Page Time is to add an EPT counter directly to Lightning Experience, which displays in the header. EPT is the time it takes to download and display the entire content of a webpage in a browser window.

1. Add ?eptVisible=1 to the end of the URL. To turn off after testing, use ?eptVisible=0.

2. Enable Lightning Component Debug Mode from Setup.

Other ways to check – https://help.salesforce.com/articleView?id=technical_requirements_measuring_ept.htm&type=5

Note:

System exception ‘ConcurrentUiRequests Limit exceeded’ will be raised when you exceed governor limitations for your Salesforce organization. By default this is typically set to 25 long running UI requests for 10 seconds. This may vary depending on your Salesforce edition (displayed in the title bar of your browser).