So, I am not an expert here - but I have two kids w/o attention issues and one with - this is what I noticed in terms of reading:
a)my ADHD dd7 learned to read by "whole word" reading. She simply did not have the focus to say in her head : cuh - ahh- tuh and then put it all together... (wait.. what was the first sound again?). It was much easier for her to just "see" that this was "cat"
b)I worked with her on "sight" word reading.. this was easy. I would write notes on post it notes and stick them on the wall by the kitchen table... This way they could be looked at frequently, but it only needed a few seconds of attention at a time. For a younger child just starting out this way I would stick with words that are visually distinct from one another "go" and "the"
c)The harder part was "comprehension" - she could read every word in a whole book now and you would get to the end and ask her to summarize it... and it would be like - ummmm I don't remember what happened at the beginning. For this, we worked with a reading tutor since she would get all pouty when I would ask her to recall - it is still not perfect - but I think she has definitely gotten better.
Reading alone for 20 minutes can be a waste of time in terms of making progress if you are not working towards a specific goal.
I think I would want more information about where exactly she is having difficulty with her reading. Decoding, comprehension, re-tell?
