Hi ankban,
Since you're given a notepad to use during the Exam, you do not have to 'memorize' what you are reading when you're dealing with an RC prompt (or any other type of GMAT prompt, for that matter). The GMAT is also NOT going to test how well you understand really complex RC subjects; when a prompt involves complex scientific concepts and/or "big" vocabulary, most the questions that will come with the passage will likely be fairly straight-forward (and more about the structure of the prompt than anything else). If you are having trouble staying focused while handling lengthy passages, then you have to consider how 'active' you are about reading the material (including how consistently you take basic notes - a task which forces you to think AS you are reading). It's also worth noting that you do NOT need to be a 'fast' reader to score at a high level on the GMAT, but you do you have to be efficient with your work (again, in how you read and take notes) to finish on time and earn a high Score.
Before I can offer you any additional advice for your studies, it would help if you could provide a bit more information on how you've been studying and your goals:
Studies:
1) How long have you studied? How many hours do you typically study each week?
2) What study materials have you used so far?
3) On what dates did you take EACH of your CATs/mocks and how did you score on EACH (including the Quant and Verbal Scaled Scores for EACH)?
Goals:
4) What is your overall goal score?
5) When are you planning to take the GMAT?
6) When are you planning to apply to Business School and what Schools are you planning to apply to?
GMAT assassins aren't born, they're made,
Rich