I got this feeling as well and I did 50% better with the plain Literata. I wonder the bolding might speed up slower readers and slow down faster readers by encouraging a word-by-word pace. It would be interesting to see a scatter plot of Bionic vs Literata speed.
Yes, I think some of the popularity with Bionic reading is for ADHD users who I believe are slow readers in part because they have to go back and reread because they lose focus. I am one of them, and according to this, I am 15% faster with Bionic reading.
