Hi Fishimura,
To start, the scoring algorithm on the Official GMAT is far more complicated than most people realize. Since that algorithm is proprietary, no GMAT company has an exact match for it, thus CAT scores can vary a bit based on the 'biases' involved in their respective designs. Beyond that issue, there are a variety of factors that can impact how you perform on a practice CAT/mock, including the time of day that you take the Exam, the order of the sections, how well-rested you are, the testing environment you're in, whether you have seen any of the questions before or not, etc.
In addition, many Test Takers face pacing issues in the Quant section, the Verbal section or both - so you are not alone. However, it's important to remember that pacing problems do NOT exist on their own - they're the results of OTHER problems.
Before I can offer you the specific advice that you’re looking for, it would help if you could provide a bit more information on how you've been studying and your goals:
Studies:
1) How long have you studied? How many hours do you typically study each week?
2) What study materials have you used so far?
Goals:
3) What is your overall goal score?
4) When are you planning to take the GMAT?
5) When are you planning to apply to Business School?
6) What Schools are you planning to apply to?
GMAT assassins aren't born, they're made,
Rich