The method I have used so far is to take the first passages keeping time and trying to move quickly (no more than ~10-12 min a passage or ~2 minutes on any one problem) but allowing yourself plenty of time to analyze and discover the answer yourself. Then after maybe a week or two (20-30 passages) start to be a bit more intentional on the timing, such as a group of 3 passages in 25 minutes (~8 min each). Slowly working towards 6-6.5 minutes by halfway or farther into studying. I have been doing this for several weeks and have done several passages under 6 minutes while the tough ones are still around 10 minutes (starting at 11-12 minutes per passage), and I'm averaging about 8 minutes now with months left to continue improving.
I think the main key is to slowly speed up and, even more importantly, allow yourself time to develop the thought process for MCAT problem solving. This is NOT the same as getting problems wrong by going fast and then looking at the right answer. This involves figuring out how to think passage style and slowly speeding this process up after discovering it.
If you're spending 3 months studying this seems like a reasonable goal:
1st month- go from keeping time to 8 min or less / passage
2nd month - go from 8 min to under 7 min / passage
3rd month - (full length tests taken) keep working towards 6 min / passage