Once you get into it it's not any slower than typing; if anything I think you can actually be faster with a good enough grammar, and it's precise too.
The problem I had eventually was that dictating all day is itself something that wears you out, so you get hoarse and honestly a bit more tired from it when you do it too much.
But no, overall a good Dragonfly grammar should be something that keeps you at least up to par with your typing speed.
