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iOS 9 is on 6 out of 10 of Apple's mobile devices

The adoption rate for Apple's latest mobile operating system, which the company called its fastest iOS adoption ever, has slowed.

iOS 9 adoption has slowed in recent weeks. James Martin/CNET

Apple's iOS 9 adoption rate has leveled off following an initial spike when the software was first released last month.

As of Monday, Apple's newest mobile operating system is on 61 percent of all iOS devices, which include iPhones and iPads, according to Apple's App Store Distribution page. That's up just slightly from the 57 percent rate seen on October 5.

After rolling out on September 16, iOS 9 surged to an adoption rate of more than 50 percent just four days later. That prompted Apple to proclaim it as the fastest iOS adoption rate ever.

The slowdown in iOS 9 adoption comes as the company has dealt with a number of technical flaws. Just as iOS 8 was plagued by technical issues early on, iOS 9 has been the victim of several flaws, some more significant than others.

The problems have forced Apple to release two bug-fixing updates so far: iOS 9.0.1 and 9.0.2. But problems persist, at least for some users, even in the latest update, Forbes reported on Monday. Since October 12, developers have been testing the fifth beta release of iOS 9.1, which is designed to clear up all the nagging bugs.

iOS 9 adoption still outranks that of its predecessor, according to Apple's numbers. Released on September 17 of last year, iOS 8 achieved a 46 percent adoption rate on September 21, 2014, as measured by the App Store. But then after that initial surge, the pace quickly started to drag, and it took until October 20 for iOS 8 adoption to climb past the halfway point, reaching 52 percent.

Apple's stats include existing devices that upgrade to iOS 9 as well as new devices that come preinstalled with the latest OS. Third-party analytics firm Mixpanel, which bases its data on mobile-app usage, gave iOS 9 a 64.7 percent adoption rate as of Monday. Fellow analytics firm Fiksui was more in line with Apple, giving iOS 9 a 61.6 adoption rate.

Apple did not immediately respond to CNET's request for comment.