I remember one time when I was in high school I participated in a science quiz / competition. A few weeks later, the results were returned. My overall score was poor. The results included the correct/incorrect assessment for every single question, and looking at this, one could observe a curious thing:
Almost all of the first third of my answers were correct. Then the other two-thirds were nearly all incorrect. One could see this very obvious point at which the answers changed from being nearly all correct to the opposite.
You can guess what happened at that point. That was when I hit the time limit. When doing the quiz, naturally I started at the first question, and sequentially proceeded to answer each question. About a third of the way through, I was interrupted -- that was the end of the time limit. So I was forced to just randomly guess the remaining two-thirds of the answers without even reading the questions, no more time.
So I failed the quiz, but only because of this stupid time limit. The trend in the results indicated that I would have done quite well if I had been allowed to finish the damn thing in my own time.
It was demoralizing and discouraging. Before that, I was a very good student in my science class, but then after that stupid quiz, I became less interested in my science class and less motivated to do the work. Only as an adult do I now understand that it was an unfair test and I had no reason to feel demoralized.
Teachers need to understand what it feels like when you work hard in a class, you are confident that you know the subject very well, you believe you are a good student because of how well you have been paying attention in class, and then you do an exam or quiz which tells you that you are crap!! That hurts, and as a kid you do not realize that the exam/quiz was unfair and is an inaccurate reflection of your abilities.