Its just activity, see the formula you linked. Its boild down to 3 basic steps:

the first major reduction to 75-30 seconds once your activity is over 14, but still not 60 or higher
the second reduction to 30-27 seconds once your activity is 60 or higher, but still not 100 or higher
the third reduction to 12-4 seconds once your activity is 100 or higher.

Once you reached 500 actitivty you will be stuck at a 4 second limit, which I notice once in a blue moon.