However this is considered “slow” compared to previous iOS releases. For example iOS 10 managed to hit an install base of 14.45% after the first 24 hours of release, while iOS 9 managed to hit 12.6%. As MacRumors notes, iOS 11 does not seem to have run into any installation issues unlike some previous iOS releases, but it seems that those bad experiences might be the reason behind the slightly slower adoption as some users might be holding out until the reviews are in, and any potential issues might have been addressed.
While iOS 11 is hardly a bug-free release, so far nothing major has been reported save for sending emails using the Mail app if you’re using Outlook.com or Exchange, but otherwise the update seems to be doing just fine. Also what’s worth noting is with iOS 11, Apple is dropping support for 32-bit apps.
This means that there could be some apps that are older and no longer updated that users still rely on which could be why they might be holding out from updating as well. It is possible that things could pick up later, especially with the iPhone 8, 8 Plus and the iPhone X being released.