If comprehension of material is the issue slowing yourself down in the RC passages, then try doing more RC questions and start learning how to skim over extraneous material and get the gist of the passage. It's not reading skills in and of themselves that the GMAT tests, it's the ability to take in a lot of information and understand how/where to quickly look back to in order to find the information requested in the questions.
So read passages and keep a sense of where you get bogged down. Are you reading every single word or re-reading sentences? If so, try to keep yourself moving along, since you're wasting valuable time. You can look back on the passage when you get to questions, you don't need to memorize material, especially on the complex scientific passages. Just visualize where bits of info are so you can find them when you need them. If PG 3 talks about the hadron collider, then just skim over it and know to look back to PG 3 if a question asks about a specific detail on the hadron collider.
Also try to read businessweek and scientific american, but ask yourself questions as you go - what was the main point of this article? Why did the author mention this interview? Train your mind to be active as it reads and pick up information automatically.
But the #1 piece of advice is practice practice practice. The more you read and the more you're able to shave seconds and eventually minutes off your total time, the easier it will become.