[gnicholas]

There are several independent (but not peer-reviewed) studies, mostly done with students. In that context, reading fluency gains can be up to 100% in the course of a month. But for adult readers the max would be around 30%, with the average around 20% and the median a bit below that.

For some people, speed is the only thing they care about, and I'd agree that 3% isn't a huge bump. But for others, the primary benefit is ease of reading, especially for long periods of time. This ease benefit is subjective and therefore hard to measure (vision scientists tend to look at blink rate, but this is an imperfect proxy), but in our experience about 85% of readers find it easier with the colors. Interestingly, this varies by device size, with a more pronounced effect on smaller screens.