A little stress can help perform a task because it heightens awareness, but too much stress degrades performance.
The optimal amount of stress/arousal depends on the difficulty of the task. Difficult tasks require less arousal while simple tasks require more arousal to keep the user's attention.
If people are performing a boring task then you need to raise the level of arousal with sound, colors, or movement.
If people are doing a difficult task, then you need to lower the level of arousal by eliminating any distracting elements.