FILE - In this Jan. 17, 2014 file photo, Apple's CEO Tim Cook, left, gestures as China Mobile Chairman Xi Guohua smiles during a promotional event that marks the opening day of sales of China Mobile's 4G iPhone 5s and iPhone 5c in Beijing, China. Soaring sales of iPhones in China, Russia, India and Brazil during the April-June 2014 period helped Apple overcome softening demand for the device in the U.S. and Europe, where consumers seem to be more interested in waiting for the autumn release of a new iPhone that's expected to feature a larger screen. (AP Photo/Alexander F. Yuan, File)

Ever wonder why your iPhone seems to slow down after a few years? Why the once-amazing device gets cranky and struggles to perform basic tasks or load apps?

The answer lies in Apple's software, and it's a key part of the company's strategy to keep millions of people buying new iPhones.

Apple releases a new mobile operating system every year, and that keeps a powerful cycle in motion. Each fall for the last few years, people have rushed to download the latest and greatest version of iOS, which is designed for -- and, as a result, works best on -- the newest hardware that is also released around the same time. In the months leading up to the release, many app developers furiously update their apps for the latest operating system.

Here's how that affects you: If you have an iPhone that's more than two years old, and as Apple recommends, you've upgraded the operating system a couple of times since you bought it, you may find yourself wanting to throw your phone against a wall. It's likely gotten slow and finicky.

For many, the solution is simply to buy a new iPhone.

It's highly unlikely Apple deliberately slows down older iPhones just to get you to upgrade. The company declined to comment for this story. Instead, Apple designs the new operating systems, which have more features, take up more space and require more computing power, for the new iPhones. And a consequence of that is they don't work as well on older iPhones.

The system has been pretty successful for Apple. iPhone owners in the U.S. tend to shell out big bucks for a new iPhone about every two years (which, not coincidentally, is also the length of the traditional wireless contract.)

But with its latest update to iOS 8, Apple hit a few bumps.

Still, a huge number of people rushed to download iOS 8 in the first few days it was available.

Justen Meyer, a 33 year-old who works in the pro sports industry in St. Louis, was one of those people. He regrets updating his iPhone 4S, which he says is now "slow."

"It's horrible. My apps don't work. Twitter won't open," he said in an interview recently.

Before the update, his phone was "perfect," he said. "I was completely happy. Now it's making me wonder if I'm going to go through this the next time I get a new phone."

Meyer isn't alone. People complaining about their iPhones feeling slow after new iPhones and operating systems come out is nothing new. Catherine Rampell wrote in The New York Times last year that her iPhone 4 felt "a lot more sluggish" after the 5S and 5C were released. Sendhil Mullainathan, a professor of economics at Harvard, noted in another Times story this summer that Google searches in the U.S. for "iPhone Slow" spike when each new iPhone is released.

Part of that could be because so many people download the new operating system at the same time, iMore Editor-in-Chief Rene Ritchie pointed out earlier this year. Apple releases its new OS to everyone at the same time, while Android updates hit different phones at different times. (This is one of the reasons why Android's operating system is so fragmented -- only a quarter of Android owners are on the latest version of the operating system.)

iPhone models that are a year old -- the next-to-latest generation -- tend to do fine when they're upgraded to the latest operating system, said Mike Gikas, who as the senior electronics editor at Consumer Reports has reported on the testing of every iPhone that has come out.

It's the older ones that begin to lag, Gikas said.

"We do know as the phone becomes older, the apps seem to become sluggish and some of the capabilities diminish," he said.

Indeed, Twitter is full of people complaining about their phones slowing down after downloading iOS 8.

finally got the iOS 8 update & I'm pretty sure it was more of a downgrade. phone has never been so slow or unresponsive. #rant 📱🔫 — travis (@originaliowaguy) October 6, 2014

Ios 8 was specially made to make my iphone5 super slow. 😪 — shayiful. (@ShayifulEusoff) October 6, 2014

New software has been slowing down older hardware since before we were jamming CD-Roms into our Gateway 2000s. What's different with iPhones is the sheer breadth of third-party software that runs on the phones. Apple has 1.3 million apps in its App Store. And in order for those apps to remain relevant and keep working, developers have to update them to work with Apple's latest software and hardware.

Apple aggressively pushes developers to design apps that work on the latest operating system, said John Poole, the founder of Primate Labs, a Toronto-based company that makes apps to measure the speed of smartphones.

"Apple encourages developers to target the latest version of iOS by providing tools and APIs that only support the latest version of iOS," Poole wrote in an email to The Huffington Post. "For example, it’s difficult to write an app that supports both iOS 7 and the new iPhones' screen sizes."

Poole added that Apple's huge publicity machine that is the App Store currently promotes apps and games for iOS 8, which provides a further incentive for developers to design for the latest and greatest.

Of course, Apple wants to develop its software, too, and it isn't looking backward to make sure older versions of the iPhone work perfectly with newer versions of iOS. In order for Apple to remain a leader in the highly competitive smartphone game, it needs to innovate and focus on optimizing people's experience with the newest products.

"Apple faces that choice: Do they create an operating system that works well on older devices, or do they create an operating system that takes advantage of the latest hardware?" Poole said. "Apple's focus has been to make a compelling operating system that takes advantage of the latest phones rather than an operating system that will work well on older phones."

And therein lies the conundrum -- or, if you're Apple, the genius: You want to download Apple's latest software since that's what developers are developing apps for, and it comes with new features and important security fixes. But if your phone's more than a year old, it may slow it down.

Christopher Mims, a former technology and science editor for Quartz who's now a technology columnist at The Wall Street Journal, has been using Apple products for more than 20 years. In a story for Quartz last year, Mims explained how he's found a way to get more time out of an older iPhone: He simply chooses not to download the latest operating system.

"Except for small, incremental updates, do not upgrade the operating system of the hardware you are using," he wrote. "Especially if it’s currently getting the job done."

Mims now has an iPhone 5S, last year's flagship phone. And he has no plans to download iOS 8.