After introducing Siri Shortcuts for iOS 12 — a way for users and developers to teach Siri to do more than just fumble for directions and songs — Apple notably left the feature out of iOS 12 betas, later releasing it as a standalone app for developers to test. A Redditor appears to have discovered the reason: The Siri Shortcuts app consumes enough system resources that at least one of its features won’t work on older devices.

In an email exchange between developer Federico Trevisani and Apple SVP Craig Federighi, the software engineering chief says that while Apple always tries “to bring as many new software capabilities to as many devices as possible,” sometimes it can’t do so “without compromising the overall performance of our older devices. Unfortunately, this was the case for Siri Shortcuts on iPhone 6 and iPhone 6 Plus.”

While the email superficially appears to suggest that Siri Shortcuts won’t appear on the older devices at all, Trevisani subsequently clarified that he was asking specifically about one of the app’s most useful features, “Siri suggestions for shortcuts,” a feature he notes is available on the iPhone 7 but missing from the iPhone 6. This proactive feature is meant to surface Siri Shortcuts that are available in the phone’s existing apps — a convenience for users and developers that flags what Siri can do — but apparently the required processing degraded the rest of the iOS experience enough on iPhone 6 devices that Apple opted to remove it.

Siri Shortcuts is based heavily upon Workflow, an app Apple acquired in 2017. The interface and core functionality are similar between the apps, with both serving as ways to automate commonly performed iOS user tasks, but the newer version adds deep integration with Apple’s AI assistant and voice recognition capabilities. Despite processor-dependent feature limitations, the app is expected to offer most of its functionality on all iOS 12 devices.

Since the iPhone 6 runs on Apple’s A8 processor, it’s quite possible that other A8-based devices, including the current-generation iPod touch and iPad mini 4, will have the same limitation. That detail and others should be confirmed when the final app is publicly distributed, most likely later this month.