Apple revealed Thursday that app slicing, an expected feature in iOS 9 that would dramatically reduce the size of downloaded apps, is not yet available.

App slicing was meant to cut down the size of apps by only downloading the necessary data. Normally, when an app is downloaded it provides assets for all devices the app is supposed to run on. Images, for example, come in sizes for both normal and Retina displays. App slicing detects the device the app is being downloaded to, ignores the bits that aren't needed and cuts down app sizes to the bare essentials.

The company claims that an issue affecting iCloud backups, where certain apps would only restore to exactly the same model of device, meant the feature was not yet ready for prime time. A software update will introduce it at a later date.

The issue will hit iPhone owners with low storage exceptionally hard. Critics have complained that the iPhone 6S still comes with only 16GB of storage in the base model, considered very low for a phone that shoots 4K video. The last time Apple upgraded the base model's storage was in 2009 with the launch of the iPhone 3GS.

It has been a tough start for Apple's latest mobile update. Twitter users were in uproar on launch day when their downloads failed, and forum commentators claimed crucial bugs hadn't been fixed. But the negative publicity doesn't seen to have stopped Apple from storming ahead on download figures. On Monday, the company announced that iOS 9 has the fastest iOS upgrade rate ever, and is on course to become the biggest software release from Apple in history.