Sometimes I wonder why kids have to be timed on things in school

I was in the classroom testing first grade kids on their reading. They were actually quite good at it. Then I took them into the hall one at a time to test their skills on speed. I would give them a paragraph to read and then have them start whenever they were ready. Then I would start the timer for one minute. The kids would instantly start reading as fast (and often as loudly) as they could. But very few actually did as well as they did reading on their own in the classroom. I could tell they were nervous and stressed. Some kids more than others, but most seemed to feel the stress. I think from now on we should test them without their knowledge