We can adjust the speed of testcase execution during execution using Speed control in the Execution view as shown in the screenshot.
By default we cannot see the speed control slider
We have to follow these steps to display the Speed control slider
1 Click Window > Preferences.
2 On the Preferences page, click Spirent > Views > Execution View.
3 Check Display the execution speed slider in the Execution view.
These settings to give us a chance to observe behavior that you might miss at a higher execution speed.