At the rate this is going, it is very possible that iOS 9 could hit more than 20% within 48 hours. That is a very impressive start. It has been suggested that the reason why it was slower right out of the gate was due to issues downloading the update. Several users reported that they received error messages when trying to download it.
It was attributed to the rush of downloads on Apple’s servers causing the error. However if you do receive the error even today, you might want to try manually downloading it via iTunes and installing it from there instead. That workaround seems to have the most success for a lot of users, plus it’s great if you’re a little low on space.
All signs seem to be pointing towards iOS 9 potentially eclipsing iOS 8 in terms of adoption rate. This might have to do with some pretty major new features, like an updated Spotlight and Siri suggestions that feels a lot like Google Now, along with the new battery saving features that could potentially allow users to get an extra hour or two out of their device.