We do go to great lengths to track the time“button-to-button" very accurately. For example, when you do a group start or stop, we remember the button time and provide that same time to each runner, so even if it takes a moment to refresh the screen, all will have the same correct time. Rest assured that the lag you see is purely visual.
However, just like any manually triggered stopwatch, the problem is the ability of the person using it to see the person coming to the line, find the right watch and hit it at just the right time will truly determine the accuracy of the timing.
Practicing this skill, using photos in the watches to identify individuals, and trying Watches’ auto-sort feature can greatly improve this accuracy, but nothing short of a fully automatic electronic timing system will be able to capture the exact times.