like the other posters have said, before you even start working on these hard passages, you should quickly survey it to note the difficulty. if it's something you're weak on, say optics, i would save that one for later instead of wracking your brain. psychologically speaking, you'll know you're not in too much trouble because you've done most of the other questions, and you won't blank or start glancing at your watch obsessively.
if it's verbal, then make sure you read the first sentence slowly to get a grip on what you're reading; otherwise, you'll get halfway through the passage and not even know what it's about. your speed should increase after you get that gist.
as for the PS and BS sections, rest assured that a lot of the questions from the passages require only a minimal glazing over the passage. just make a quick note when reading it about any relations, like what is proportional to what, or any outside knowledge. for instance, if the passage starts going on about photons, i would write out the formula E = hf.
you'll be glad you only made short notes because when you come to the question stems, you'll see that the questions don't rely heavily on a close reading of the material. try it.
