MGMAT are generally considered more difficult than the GMAT. More people mention this about the quant section more than the verbal section. Overall, the six online tests are good for test practice and strategy. The only thing better is, naturally, the GMATPrep.
For timing strategy, read this thread:
timing-strategies-on-the-gmat-80176.htmlAlso, make sure to look away from the screen sometimes. Staring at a screen for over three hours can burn you out. During the test, I would even close my eyes for a moment while taking a deep breath. This helped me stay fresh for as long as possible. Another way to help with your speed is to take the breaks during the test. Speed and energy are related, so eat a small snack between the sections. And no matter what, stay calm during the test.
If your'e still practicing with questions (outside of the CAT's), make sure to time yourself with a stopwatch or the
GMAT timer. I prefer the latter because it's free and, since it's on the computer, forces you to practice problems in a setting similar to the test center's environment.