Because a slow (but secure) device is a continuous and frustrating reminder that you need to buy a new phone. Whereas a fast (but unpatched) phone is a joy to use, except in the unlikely event that someone actually hacks you, and then it's not clear whether you would go buy another Apple device. I'm thankful that Apple is doing the right thing by patching older devices, but I also think it aligns with their financial incentives.

And to be clear, I don't subscribe to the theory that Apple is intentionally slowing iOS down on older devices. But subjectively, the iPhone 4S struggles with iOS 9 the same way that the iPhone 5s struggles with iOS 11. Even if you work in IT, it's hard to fathom that iOS 11 looks and works the same as iOS 9, but requires a ~4x more powerful CPU to barely chug along[1]. Planned Obsolescence almost wins Occam's Razor here; I'm not surprised that so many people misinterpret the battery throttling thing.

[1] https://browser.geekbench.com/ios-benchmarks

Interesting. The reviews at the time claimed that it was slower, but not slow.

https://arstechnica.com/gadgets/2017/09/ios-11-on-the-iphone...

I've seen these okay-ish numbers, but subjectively it was a crap experience (bad enough to buy an SE). It wasn't really app launch times that bothered me, though. Scrolling all around the OS "felt" (not sure how to measure it) like it was running way below 60 FPS, whereas everything was buttery smooth on iOS 10. And the OS seemed to only keep one or two apps in memory, ever.

I suspect it might be because I only had 1GB of free storage, and iOS 11 was uploading and downloading cloud photos like crazy? Often I would take a picture, and by the time I wanted to send it via message ten seconds later, the phone had deleted the original it from my phone, and had to re-download it.

I would gladly assume that I'm an outlier, but a friend with a 5s asked me for new phone recommendations right after updating to iOS 11, so there's at least two points of anecdata.