Most gmat test-takers find themselves under a lot of time pressure in the real test. With only about two minutes to answer each question, it is important to adopt quick and efficient approaches to problem solving. Some can answer most GMAT questions given four minutes; if this is your problem, you need to accelerate. There are many techniques that help to do mathematics more quickly. Most GMAT test-takers can even learn to do simple arithmetic more quickly - many have grown accustomed to doing calculations in a spreadsheet or on a calculator, and need to review how to do pen-and-paper math.
The gmat is also an endurance test. A complete test lasts three and a half hours, and you'll need to ensure you can maintain focus for the duration.