The command actually takes effect instantly. The response from the device to your session screen is what causes the appearance of a slight delay. If you send a lock command to a student’s device the browser will be immediately blacked out.
To verify if the command was sent to the device you can select the commands tab, where it will show you the commands that were used, and if they were successful or not.
For more information please visit our 'Interacting with Chromebooks' article here.