This topic comes up a lot, and the "perfectionism" explanation is a common one (assuming it's not a legitimate case of slow processing). I'm not convinced. I think the problem is that you have a task where you need to come up with a strategy to find the right balance between speed and accuracy, and you have no reference point to know how fast you should be trying to go (and maybe don't have the life experience to even think about such strategies) so you simply set about the task most likely at a non-optimal speed.

I just don't think it's a good test where the score depends on whether a child happens to hit on the right balance of speed and accuracy.