Characters per second
A 17 CPS ceiling, and a cue that fails it on purpose.
Reading speed here is characters divided by seconds on screen. Spaces count; line breaks do not add a character. The default ceiling is 17 characters per second, a common broadcast guideline. It is not a law of any one app.
The sample cue “Fast” runs from 00:00:01,000 to 00:00:01,200. That is 0.2 seconds and 4 characters: 20.0 CPS. It fails a limit of 17. “A readable caption.” is 19 characters over 3.0 seconds: 6.3 CPS, which passes.
checker lists every cue over the limit you type. Raising the ceiling hides problems; it does not make them easier to read. timing fixer can close overlaps, but it will not invent time that the next cue already occupies.