Just a thought - if you come across a tough question, do you keep solving it till you really get it even it it takes you a really long time or do you look at the answer and move on even if you don’t completely get it? I feel like I am wasting a lot of time on certain questions. What do you suggest?
On exam day, if I can’t do it in 3-4mins then I’ll put my guess answer down, circle it in the book and come back to it when I review the paper. Don’t waste heaps of time on one question unless you’re cruising through the paper. You won’t know every answer, but can make some pretty educated guesses.
And what about during your practice? Do you do the same? Do you move on after 3 - 4 mins. I don’t know how much time to spend on tough questions. And then there are questions where I know the stuff really well but somehow can’t get the right answer so I keep solving it. Turns out to be a careless mistake in the end 
Yeah I move on, although once I’ve finished a mock I won’t go back and review answers before I grade it, I’ll do that to the questions I got wrong. Remember it’s 3mins per Q, so it’s good to try and train yourself to finish well within the time.
I would start by POE - removing one probable wrong answer is much better than leaving it to wild guess. Mark it in my booklet and move on. If it is a calculation question, I’ll make sure I have my steps clearly labeled, so I won’t have to start fresh if I have to revisit the question. I’ll highlight what I know, what I don’t know, again for the same reason. When I reach the end, I revisit those question - worst comes to worst I’ll guess based on what I believe makes the most sense, and whatever that my POE earlier cannot eliminate. As others have clearly pointed out, do not spend way too much time on one question, even if you know how to do it. For the same reason, I would mark long calculation questions for later.
Thanks!