In the case that the system is working as expected, it'll only wait the 1 second for the implicitly_wait to return (note that this is important for webdriver remote only. For local webdriver implicitly_wait is set to zero). 

A couple statements up we already did check that the error msg is shown and the iframe is not visible so all synchronization is handled there.
 
We can set that wait_time of the iframe.is_element_not_present_by_css to 30 seconds if we want. In the success case it would still return right away. In the failure case it would keep polling for 30 seconds. Thus the wait_time=1 will be quickest for either condition.