Depends on the rocker. I don't like to estimate my speed as when the adrenaline bites or even before, everyone thinks they are going much faster than they really are. In reality I doubt anyone reaches 30mph down Pentonville Rise, but everyone thinks they are doing more than 40. Unless you were clocked, with a heavily rockered setup, you were probably doing somewhere between 15 & 20mph before the wobble set in.
I have GPS clocked myself rock steady over speed bumps at a whisker over 43kph with a flat setup but rocker my back wheels and I don't need to go anywhere near that fast to get unstable. I have had the wobbles down there twice, but only when my rears were rockered. Even worse than the speed wobble I got down the hill at Le Mans when I skated down there with a rockered setup. Le Mans was uncomfortable but on the street was just stupid especially without any armour. I'd say it is possible to get the wobbles at 12mph with an extreme setup but any normal setup probably won't be affected.
Would wheel size make a difference? Not sure except that bigger wheels go a bit faster so you are more likely to get it with big wheels in the same rocker profile. Wheelbase too.
If anyone is interested, the cure for speed wobble, if you don't want to slow down, is to stop gliding, i.e. start striding. Yes, trying to push yourself to go faster will make the problem go away as it changes the dynamic of the skate. However, if you get that extra speed and start to glide again you may well be doomed to a dose of road rash or worse.