Remember that on the GMAT, you *do not* need to get the majority of the problems correct. If you are having timing trouble, the first thing you need to do is commit to stay on time, even if that means guessing on some problems. Even at the 700 level, you should expect to miss about 40% of the problems, so it's no big deal to let a problem go to save time. CR are the best problems to guess proactively on, as you don't want to skip a whole RC passage, and skipping SC generally saves you less time. If you see a really ugly CR, or if you find yourself stumped by an SC problem, guess and move on. It won't hurt your score; in fact, it should help! Those are problems you're likely to miss anyway, only now you'll have a 20% chance of getting them right (if you guess randomly) and you'll have extra time to devote to the rest of the test. This is much better than trying to race through all the material, and lowering your overall accuracy.
Of course, while you're studying, you want to go back to these "killer" problems and work to understand them. But you also want to practice identifying the toughest problems, so you know which ones to drop (if need be) on test day.