What Threshold value?
Performance threshold is the maximum acceptable value for the metrics identified for your application/test case. If your application's performance is more than the threshold value, you will be notified through email.
How to set Performance Threshold value
1. Navigate to Manage Synthetic Tests and monitor a test case and wait for the result
2. After successful execution, you can configure Threshold value, click on more options in Actions Pane
3. Input the threshold value and save