Extra time spent getting a hard problem right will hurt you even if you get the question right! Here's why: If you use up precious time (let's say 4+ min) and get the question right the next question will be just as hard if not harder. You're essentially placing yourself in the 700-800 range with limited time which will cause you to crash and burn at the end. Not worth it.
One of my students once shared some interesting results from her mba.com practice exams. She had received a 42 scaled score on quant while missing 11 questions, but then received a 47 scaled score on quant while missing 13 questions. That's right, she missed more and received a higher score. You see it's not the # of questions right that matters, it's getting all the easy and medium level questions right while strategically maneuvering through the hard questions. Have you ever considered intentionally bailing on a couple questions in the middle of each section so you don't have to rush at the end?
