Apple released iOS 8.4 at the end of June, and after just a week more than 40% of iPhone and iPad users have updated to the latest iteration of the operating system. That’s an unprecedentedly fast adoption rate for an iterative release, quicker even than the full-point update to iOS 8.0 last year.

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It won’t be long before the majority of users are on iOS 8.4

So why have users flocked to the new update so keenly? Two words: Apple Music. iOS 8.4 includes an update to the Music app which is necessary to use Apple’s flagship new music streaming service on a mobile device. (It also includes a quiet update to iBooks.) Clearly the advertising for Apple Music – not to mention the three month free trial – is proving to appeal to many iOS users.

A secondary reason for the mass adoption may be the fact that Apple has been working to slim down the amount of storage space required to upgrade, meaning less clearing up of old photos and apps just to make room for the new version. This leads to an easier process for customers and leaves them less likely to put off the task of upgrading.

Though iOS 9 has many improvements and new features planned, it doesn’t have a big new killer service like Apple Music to drive adoption. We’d be surprised to see users move to iOS 9 as quickly as they have embraced iOS 8.4 – but we’ll find out soon enough, when it launches this Fall.

Read more: iOS 9 features preview

Adoption data and graph from Mixpanel