So you know that room temperature is about 27 C right?
At 0 seconds, the temperature is 68, then at 20 its 53
There's a temperature difference of 15 degrees
68 degrees is very far from the room temperature, so the change in the next 30 seconds is very large
But as the temperature gets closer to room temperature, the change is less.Eg:
At 270 seconds, the temperature was 28.5
At 300, its at 28
The increase is only 0.5