I also use a similar reading style as testami. I read it once and try to garner a decent understanding and then go over all the readings again and make sure I understand it all.
Also when you are doing a test or an exam you have to examine what level of stress and concentration you are under, what type of questions you are doing, etc, etc. I know schweser exam questions can be twice as long as a typical CFA level 1 exam question, and that's if you are fast, along with that there are some questions. I would say if you are getting lower it's because you don't understand the material well enough or maybe you were under heavy stress or lacking in concentration. Remember, you can't pass the CFA exam (most likely), just by memorizing the material.