I agree with praetorian...
First 15 questions are important in the sense that if you dont answer them enough of them correctly, you will get stuck in the lower score bracket .. if you answer them correctly you will be put in the higher bracket .. but you have to sustain the momentum on the tougher qs that follow (in fact you need more time for the tougher qs thereafter .. so spending most of the time in the qst 15 qs can be counterproductive sometimes) Your score may not go down as much if you answer only the 25th qs wrong .. but if you answer qs 25th to 35th wrong .. your score is certain to drop a lot ..
I had seen this in my practice tests too .. if I was doing badly on tests, I would generally finish much earlier .. and I would take much longer if I scored better on earlier qs.
-trishul