The critical path in the Gantt chart is the sequence of tasks that could delay the project end date when any of the tasks get delayed.
When this option is enabled, your cards are constantly checked, which can impact the performance of the Gantt chart.
What cards are shown in the critical path?
A card is only shown in red when a delay of the card will result in a delay of the project. As you can see in the screen recording below, updating the first card to add more slack, will remove it from the critical path.
When you set the minimum duration of a dependency, this duration is taken into account while calculating the critical path.
In the screen recording below the duration of the dependency is set to 1 day, thus including the card in the critical path. Note: you can click on a dependency to set the duration.