I take the GMAT in 5 days...what is the most efficient use of my study time?

Can't be answered in detail without knowing your current strengths and weaknesses across question type, content area, and timing, and your goal score.

Generally, though, not much new stuff gets learned in the last few days. This is the time to do high-level review, plan your strategies for all question types & content areas (actual content, timing, technique, educated guessing, etc), and acknowledge your strengths and weaknesses.

For your strengths, make sure you spend adequate time when you get a question of those types on the test - don't rush because you think, "This is easy for me." Also for your strengths, give yourself permission to spend a little extra time on especially hard problems of those types.

For your weaknesses, make sure that you do NOT go over time. You're already not good at these things - don't spend extra time getting them wrong. If you're ever tempted to spend "extra" time on anything it should only be on something you have a good shot at getting right. By definition, those are your strengths, not your weaknesses.

Good luck!