My daughters 2nd grade teacher recommended this app, which I have found very useful. She does not struggle with comprehension in reading, but does struggle with her out loud reading speed. However, I found that as I was reading along with her on the first time, I did not pay attention to the content of the story. I agree with those that said comprehension questions would be helpful, to be sure the child is not just trying to beat the clock, but understanding as well. Yet, this is a pretty awesome app for aiding in the practice of reading aloud to increase the speed.
MKMcK1 about K12 Timed Reading Practice