time.deltaTime timer increasing faster and faster

Hey, I'm trying to make a timer that increases the longer the user looks at the same point for extended amounts of time.

Only problem is that this code here:

 IEnumerator increaseInsanity()
     {
         while (isInside)
         {
             VariableHandler.timeSpentStaring += (Time.deltaTime / 60) / 4;
             yield return null;
         }
     }

Increases slowly over time but then gets faster and faster.

For example when the timer reaches 100 it takes 30 seconds but then when it reaches 200 it takes 12 seconds Etc...

Is there anyway that I can fix this or will I just have to put up with it?