Test smells can pose difficulties during testing activities, such as poor maintainability, non-deterministic behavior, and incomplete verification. Existing research has extensively addressed test smells in automated software tests, but little attention has been paid to smells in natural language tests. While some research has attempted to catalog such test smells, there is a lack of investigation into their impact on the effectiveness of test cases. In this paper, we conduct a controlled experiment with 30 participants from academia and industry to examine the impact of test smells in manual test descriptions. Specifically, we analyze whether the presence of two test smells, Ambiguous Test and Eager Action, result in (1) increased test execution time, (2) a higher number of steps needed to complete the tests, and (3) high divergency on the perceived success of the tests outcomes. Our findings reveal that an Ambiguous Test can increase execution time by up to five times and screen flow by up to seven times. In addition, if the Eager Actions are dependent on one another, there is no increase in execution time and screen flow.