You definitely should read the passages, as they often contain stuff that you'll need for answering the questions. However, the passages do often contain some material that is irrelevant when it comes time to answer the questions, so don't get hung up on a passage if you're struggling to understand a certain part, because there's a good chance you won't need to understand that part. You should be able to read through them fairly quickly; make sure you familiarize yourself with the passage contents in case you need to refer back.
i think if you are really knowledgeable and have a lot of background on the topic you wouldn't need to read the essays.
but for most people, like myself, need to read the passages. but i think skimming it will help you regardless. if you can go through the section without seriously reading the passages then you should have plenty of time to at least skim the passage for equations or such that you might not have memorized. or just get a general idea of what kind of questions to expect in that section.
good luck to you.