iOS 10 has been out since September 13 and is now running on 66.7% of devices, giving it a faster adoption rate than any other Apple software update. During the first two weeks, however, iOS 10’s adoption lagged behind previous versions. Towards the end of September, everybody began installing it.
The initial slow start and subsequent rise of iOS 10 hasn’t been entirely explained, particularly the 20% rise from September 27 to October 4. Regardless of the cause, Apple will surely be happy about a strong adoption rate.
A couple factors could be that users were slow to install due to early problem reports and that new iPhones running iOS 10 hit the market after the software’s release.