Go to the menu Settings, then choose Board settings on the left.
This feature displays a shadow around the task when a task is moved recently.
Set the value 0 to disable this feature, 2 days by default (172800 seconds).
Everything moved since 2 days will have shadow around the task.
Refresh interval for public board
When you share a board, the page will refresh every 60 seconds automatically by default.
Refresh interval for private board
When your web browser is open on a board, Kanboard checks every 10 seconds if something has been changed by someone else.
Technically this process is done by Ajax polling.