Fluency definitely impacts reading comprehension. The essence of fluency is the ability to recognize words and understand them. So that when someone is trying to comprehend something that is being read, being fluent in that language increases comprehension.
For example, if someone speaks/writes Spanish fluently, but only speaks/writes English moderately well, reading a book in Spanish will provide a more successful outcome that reading in English. Comprehension will slow down.
Leslie Pepper in "Does Fluency Affect Comprehension?" writes:
If children labor to decode words, then they do not have attention or mental resources left over to dedicate to comprehension and enjoyment, which means they are not really reading, only word calling.
The idea of "word calling" promotes a sense of recognizing the word, but not being able to string the words together so that understanding and comprehension take place. This is why when students say, "I read it, but I don't understand it," even though the reading is at their level, I suspect that reading is not truly taking place; only "word calling."
Reading comprehension is defined as:
...the level of understanding of a writing.
If the level of comprehension is not present, fluency in reading cannot take place.