Address this issue with the following techniques:
Daily scrum to review progress. If people aren't making progress at a reasonable pace it will become more obvious to everyone day by day.
If a problem is suspected the product manager or line manager can review recent stories and their points and see if there is an issue that persists over time.
Continual retrospectives to uncover the deeper issue and team dynamics that are impeding progress. There are frequently hidden issue which slow progress.
Put a low performing member on review and if they can't deliver what is consider to be the minimum, after several reviews let them go. Never a pleasant option but it should be considered an option in extreme cases.
