In general, sign sanguage users find subtitles faster and more difficult than deaf people with oral English. Profoundly deaf users tend to have the most problems with fast or complex subtitles.
Speech often happens at a faster pace than reading speed, especially bearing in mind the 17% of the UK/US population that has some difficulty reading, and bearing in mind that for people born deaf, the language in the subtitles is not their first language. There are established guidelines from the TV/Movie industry for words per minute depending on age group, including how to sub down the text to fit within the words per minute.
It is critically important to convey the same information through subtitles and captions as through audio, so reducing the number of words should only by done when absolutely necessary, and be through compacting phrases, and not through removing any information.
More information: BBC subtitling guidelines
More information: OFCOM subtitle speed research
More information: Video game subtitles – what movies can teach us