I teach first grade, and usually once a student starts to read about 60-70 words per minute they start to try to read more quickly and they are not as smooth. You can support their reading with decodable text.
This is when I start to play with the levels of their reading so that even in practice, I force students to slow down while they are reading. It's also important to note that most studies indicate that conversational speech is somewhere between 120 on the slow end and 150 on the high end. When you are reading with younger children, even 80 words per minute is still a really great rate for your student to show fluidity and start to add expression.
I've had teachers ask me about kids in second grade who read at 250 words per minute and if I have longer passages for them. The truth of the matter is, once a student is reading faster than a conversational level...they are without a doubt reading TOO QUICKLY. Remember the end goal of reading is COMPREHENSION. If they are reading outside the bounds of conversational speech, you need to slow that student down. Increase the passage level until they are reading within the bounds of conversational speech. They will build stamina and learn to read for meaning rather than to read for speed.
When students are fluid, that's where the magic happens! You can do so much more with them when they are able to attack the passage and not worry about the decoding constantly. Especially when it comes to practice, don't be afraid to give students something on a lower level to read so that they are smooth. They will enjoy reading more and will feel more confident!