What Is Reading Speed?
Reading speed is simply whether viewers can finish reading a subtitle within the time it stays on screen. The math is straightforward: text width divided by seconds on screen. Common industry caps: 9 full-width characters per second for Chinese, Japanese and Korean (18 half-width units), 20 half-width units per second for other languages. A 20-hanzi line that lasts 1 second works out to 40 width-units per second — more than double the cap, and viewers will never finish it.
First, Tell Apart Which Kind of Fast
“Too fast to read” is actually two different problems: too-short duration — the cue cuts away after one or two hundred milliseconds, usually a timing error or an editing leftover; too-long text — the duration is fine, but the line crams in 30 hanzi, and no speed limit will save it. The fixes are opposites, so figure out which one you have first.
How to Check
Open Subtitle QC and drop your subtitles in. In the report, “too-short duration” is a warning that shows the measured milliseconds (threshold: 833 milliseconds); “unreadable speed” is its own finding that shows the measured width-units per second. For bilingual subtitles only the line written in the target language counts — the two languages are not added together.
How to Fix It
Too-short duration: fixable with one click — the cue is automatically extended to 833 milliseconds, without passing the start of the next cue (a fix must not create new overlaps). Too-long text: no auto-fix — the machine cannot know which word to cut. Two paths, your choice: shorten the translation (click the cell in the translation tool to edit it), or stretch this cue’s duration (only if there is a gap before the next cue; for overall timeline trouble see Timeline Shift). Run QC again afterwards to confirm the reading speed is back within the threshold.
Don’t Overcorrect
Meeting the speed limit does not mean comfortable viewing: cramming every cue right up to 9 characters per second makes the whole film tiring. As a rule of thumb, stay around 70% of the threshold most of the time, and let dialogue-dense passages go slightly over once in a while. Subtitle blocks that flicker too often are a problem in themselves — run the full set of metrics before delivery, see What to Check Before Delivery.