I just listened to some dictation passages I had made for myself at rising speeds and tried to take them down in shorthand, to see what my speed is like.
I think 60 syllables per minute (about 45 wpm at 1 word = 1.4 syllables) is the limit of what I can do more or less comfortably. I was able to get down much of the 70, 80, 90 spm (50–65 wpm) passages, but there were always bits in between where I had to leave out words, and it felt a lot more rushed. (And I noticed that I still don’t have many abbreviations internalised.)
So. Better than the 40 spm I’ve seen quoted for longhand, but there’s still room for improvement. (And I wouldn’t pass a final exam of a shorthand course that requires 100 spm/70 wpm—and I think the UK, 80 wpm/110 spm is the requirement.)