Hmmm.... I practiced over the summer and took the Sept ACT for my first time. The time was definately a factor. The more timed practice you do on your own, the better you will do. Fortunately, I got my highest ACT score the 1st time even though I was pretty nervous.
The way I keep track of timing during the test is to distribute the sub-sections out evenly.
For example, the reading section has 4 reading passages. I know that I have about 8 minutes or so MAX to finish. Make sure you do not go over the time limit by a lot. If its a few seconds, then finish the question. Keep your brain concentrated on the passages even if they're not very interesting, it's the only way you can finish and achieve a good score.