Time is of the essence!
In our recent set of mock exams one thing that became apparent was that some children, especially the 7+ and 8+ candidate, have very little concept of time. They really struggled to complete the exam papers on time. They generally showed an inability to 'speed up' or finish off the questions even when told that time was running out. They tended to just continue trundling through at whatever pace they were working at.
I think this is actually a huge problem and may well explain why some very bright children get poor marks. All the tutoring and knowledge in the world won't help if your child only manages to complete half of every exam that is put in front of them.
What can be done? Firstly I don't think many parents actually realise just how little their child will actually do if no-one offers any encouragement or advice. Exams are a lonely place if you have never done any work by yourself. I would advise that parents take a fairly easy comprehension, story title or some maths that you feel that they can do: give the child a pencil, eraser, ruler and piece of paper. Give them some instructions about what to do and then leave them completely alone for 25 minutes. Put a timer on. Then go back and see exactly what they have done, when they don't have anyone assisting them. If they have hardly done anything (or far less than you would have expected) then you need to have a discussion with them. You need to explain that it is important in exams to work harder than usual or at 'their normal pace' and they need to practice that. They soon begin to have a feel for what 25 minutes might feel like and that is very useful for them.
They need to build up their stamina and realise that it is not ok to stop and have a little rest between questions.