What I do is take about 25 seconds to quickly go through the passage and kind of figure out the organization. So in my head I'll be like, OK para1 is intro stuff, para2 is talking about 2 types of compounds, para3 is about experiment 1, para 4 is about experiment 2, para 5 is a concluding type of sentence..etc. Then I look at the questions - if they require in-depth reasoning, I'll go back to the passage and read it at normal speed to understand it. If they are more superficial questions, just answer them straight up.
Also, I find that trying to read a passage after you have looked it over with your eyes for 25 seconds prior helps me to understand it a lot more.