Some grow like crazy, some don't -- photo or auto and relative to each separately.
I had 4 germinate within 2 days of each other. While the one that germinated first is slightly older, it is multiple nodes ahead of the others after just over 2 weeks. There's a range of growth speed in all four. Autos are no different.
It's always slow at first. Canopy size will dictate. As that increases, it starts growing exponentially faster for a short period of time.