Hi again, Irmar.
Here is one of the articles about which I was thinking. The researchers have studied three speeds (12, 16, 20 characters per second).
From the abstract…
We measured viewers’ comprehension, self-reported cognitive load, scene and subtitle recognition, preferences and enjoyment. By analyzing people’s eye gaze, we were able to discover that most viewers could read the subtitles as well as follow the images, coping well even with fast subtitle speeds.
UPDATE: I’ve just discovered that this article is the one that Irmar posted at the start of this thread. I’m leaving the link here so that it makes the article easier to find now that it is two years later.