I was going to comment to say ability to pause/rewind would be awesome. Seems like you've already done that! I also like the interaction of selecting the text you want to speed-read.
One thing that might be interesting, is a shortcut to jump to the article where the current word is.
This happens when I want to glance at a paragraph above, and don't want to rewind all the way back.
HN will probably see a lot of these in the next few days.
Edit: Some users requested keybindings and I found them useful(pause, skip, rewind, etc).