#Subtitling speed
When watching films in English, young, well-educated viewers proficient
in English are able to follow #subtitles displayed at a speed of up to 20
characters per second (cps). They prefer text to be less condensed in English-language films https://avt.ils.uw.edu.pl/files/2018/07/SURE_Project_Report.pdf

When watching foreign films in an unknown language, most people prefer
dialogue to be condensed and subtitles displayed at a slower speed.
Subtitles displayed at slow speed (12 cps) result in more re-reading.

When watching English-language films with slow subtitles (12 cps), viewers
tend to notice discrepancies between the dialogue and the subtitles,
resulting from text reduction.

Subtitle segmentation (line breaks)
Linguistic units should be kept together and should not be split across
the lines.
Non-syntactically segmented text in the subtitles may increase cognitive
load and induce more re-reading.

Subtitles should not have more than two lines.
(And my favorite:) It is more important to keep closely related linguistic units together than to create a certain subtitle shape, like a pyramid or rectangle.