The charge that iOS updates deliberately slow down older iPhones gets regurgitated every so often. It's a sensational charge so it gets sensationalized. Most recently some research into correlation vs. causation in big data analytics and how it shouldn't be misinterpreted was — wait for it — widely misinterpreted. Whether regurgitators failed to read all the way through the research, or simply decided to ignore it in service of sensationalism doesn't really matter. The resulting misinformation and confusion is the same. So, what's really going on with iOS updates and older devices?
The research in question, referenced by the New York Times concerned the frequency of "iphone slow" in search queries and how it spiked at the time new iPhone hardware was released. By contrast, searches for "samsung galaxy slow" didn't spike at any particular time.
Why aren't there the same search spikes?
Why searches for "iphone slow" spike one a year and "samsung galaxy slow" don't is easy to explain. Apple owns and controls both the hardware and software on iPhones. Major iOS updates typically arrive once a year, a couple of days before new iPhones are released. Apple provides the updates directly, all on the same day, all around the world. That's hundreds of millions of devices all getting the latest version of iOS, all at the same time.
Samsung, like other Android manufacturers, owns the hardware but not the software. They can only provide updates indirectly and in a scattershot manner. They need to wait for Google to release the code, they need to decide if they're going to support it and on which devices for which carriers, they need to make their customizations on the devices they do decide to support, and they need the carriers to agree — carriers that typically prefer to sell new phones rather than update old ones.
Since Apple is updating almost everyone at the same time, searches will all be performed around the same time. Since Samsung and other Android manufacturers are updating only a few at a time and at different times, searches will be spread out across a very, very wide range of times.
In other words, someone with an iPhone 4s on Rogers in Canada is on exactly the same update schedule as someone with an iPhone 5c on Orange in France is on the same schedule as someone with an iPhone 5s on Verizon in the U.S. However, someone with a Galaxy S3 on T-Mobile in Germany is almost certainly on a very different update schedule than someone with a Galaxy S4 on AT&T in the U.S., never mind someone with an HTC One Mini on Vodafone in the U.K.
Why do people search for 'iPhone slow'?
There's a phenomena that occurs when you buy a new color car. Suddenly you start seeing that same color much more often than ever before. It isn't that everyone else went out and bought that same color as well. It's that you've become more sensitive to that color — you notice it more. And perception, as the old saying goes, seems like reality. Likewise when you update to a new version of iOS, you know you're changing things and you become more sensitive to change. You're looking for what's new and what's different so any little tick, pause, or skip tends to stand out.
And there's likely to be some, because iOS updates change things. All sorts of background tasks run during and immediately after an update. Date is restored. Apps and media are re-downloaded. Indexes are rebuilt. Libraries are migrated. That can consume a lot of resources and impact performance for a short period of time, compounding the perception.
New versions of iOS also add new features, like background refresh in iOS 7. These can also consume more resources when, for example, they're downloading new content or updating timelines. Since that never happened before the update, but does happen after the update, again it can compound the perception. (New features can often be turned off in preferences if you don't want to make use of them.)
Third party apps sometimes don't update as expeditiously as we'd like either, and some aren't great citizens to begin with. If and when they slow down or otherwise misbehave, it's easy to blame the operating system.
There can also be legitimate issues new versions of iOS running on older hardware. iOS 4 running on the iPhone 3G and iOS 7 running on the iPhone 4 both had issues at first. Rather than trying to get people to upgrade the hardware, however, Apple went into high gear and pushed out updates that greatly improved performance and made owners of those older devices happier.
What if Apple didn't update older devices?
If Apple didn't update older devices, instead of being blamed for overloading them to force updates, they'd get blamed for withholding features to force updates.
That was exactly the charge back in 2009 when Apple didn't provide video editing for the iPhone 3G because they believed performance — 15fps vs. 30fps — wasn't good enough.
It's a no win situation when it comes to conspiracy theories, but it can be a win-win when it comes to customers.
Why is updating older devices important?
The two most important aspects of software updates for older devices are security patches and compatibility with new apps. When older devices get the latest version of iOS, they get all the latest updates for the Safari browser and related web viewers, and they get the ability to run all the new apps that will be updating in the App Store.
iOS is a security-first operating system. Apple has gone great lengths to keep iPhone customers safe from common forms of attack. However, new forms of attack will always be discovered,so getting those updates out in a timely fashion, for as many devices as possible is incredibly important for maintaining that security.
Likewise, because Apple updates are so well distributed, at the time of this writing 90%+ of iPhone and iPad customers are currently on the latest version. Developers who want to use new features can adopt them quickly. Apple updating older devices means you can play that next hot game or use that next big social network, even if they're using the latest technology.
Phones and tablets that never get updated avoid the potential for slow down, but they also avoid getting new features, security updates, and the ability to run apps that require those updates.
What's in it for Apple?
Providing updates for older devices is a lot of work for Apple. Despite the size of their bank account they still face resource constraints. Nobody can do everything all the time, not even Apple. It costs time and money, it takes engineers and quality assurance, it demands support before, during, and after to update each and every version of every older device.
Yet Apple chooses to provide those because they believe it increases the value of those older devices. They believe an iPhone or iPad that gets updated for 3 or 4 years is more valuable than one that gets updated for only 1 or 2 years, or than one that never gets updated. That value translates into customer loyalty, into people who will buy another iPhone or iPad again in the future.
If Apple or any manufacturer deliberately slowed down older devices they wouldn't incentivize people to buy new devices. They'd incentivize people to buy different devices.
The bottom line
The perception that an old iPhone with a new update runs slower is completely understandable for all the reasons outlined above. No process is ever perfect and sometimes specific updates or individual devices will run slower. Sometimes waiting a little while, changing the Settings, updating or changing apps, or doing a clean re-install will fix it. Sometimes Apple will fix it.
That's how updates work in the modern software era, mobile, laptop, and desktop alike, on iOS and every other OS.