When the FPS slider is moved to decrease the FPS of a sorting algorithm being visualized, totalDelay and visualizationTime become out of sync (totalDelay > visualizationTime, which is not possible), leading to the value of Sorting Time being negative. Quick sort isn't THAT fast :)
Related to this issue, when the delay is very high at the beginning of a sorting visualization (500ms delay), the sorting time starts out initially negative.