They have now said they've tweaked the algorithm so that read time has a larger impact, which I take to mean more impact than measures of engagement. But when people click out in less than 30 seconds it reduces the read ratio, which is part of the earnings calculation. Exactly how it all works is still a bit of a mystery. :)

With everything else being the same, a writer will be paid more for a story with a 90% read ratio than the same story, with same read time and engagement, getting just a 50% read ratio.

The difference between the 90% and the 50% are the number of people who don't stick around for longer than 30 seconds.