I'm not successful much at those, but when I am it is because I have circled the difficult parts. I think it helps me remember where the parts are in relation to my pencil marks, so I can find or remember parts of the passage when going back.
I would say that depends on the proctor... Ours was insanely fast... Best tip I can suggest is when they say pencils down, pencil down and right then move your watch.
I recommend doing a lot of practice with the "tri" pencil if you are going to use it. Great for bubbling, but I did not like it at all for extended use on the essay.