Generally you design your cycle times so that the median performer never is late. Remember if each median performer has 1 delay per hour, on a line with 100 workers that mean each cycle there is a delay somewhere and so the line always moves slower than expected. How much slack to add is a complex question, but you have to assume there will be delays. The 10th percentile cannot be allowed to delay the line, so you have to deal with it (training, fire them, give them faster tools, or switch to a station they can do in time come to mind, there may be more)