If you follow Apple news closely, at some point in the last week you've probably seen the graph above. It's from Apple's Developer Support page, and the company calculates the figure by looking at the iOS versions of devices accessing the App Store. Like Google's analogous developer dashboard for Android , it's meant to give developers a broad look at OS usage so they can use that data to determine which OSes to support with their apps.

The problem with the graph above isn't that it shows iOS 8 and iOS 7 with the same amount of share, but that the number for iOS 8 has climbed just a single percentage point since the last measurement was taken on September 21. Apple's data mirrors what a number of other independent firms have been claiming virtually since launch day—Chitika's data shows that iOS 8 had rolled out to 7.3 percent of the iOS userbase after 24 hours of availability, while iOS 7 had already hit 18.2 percent in the first 24 hours after its launch. More recent data from Fiksu shows an adoption curve closer to iOS 5 (the last version you needed iTunes to upgrade to) than to iOS 6 or iOS 7.

Though the Ars audience is generally more tech-savvy than the general populace, our own data shows that you guys are embracing iOS 8 less enthusiastically than you picked up iOS 7. Here's data from iOS 7's first two full weeks (running from September 22 of 2013 to October 5) compared to data from iOS 8's first two full weeks (September 21 of 2014 to October 4). Around 70 percent of our site visits came from iOS 7 in that time period, compared to about 60 percent from iOS 8.

Hitting 47 percent after three weeks of availability is a feat that Android or Windows or even OS X can only dream of, but after the swift uptake of iOS 6 and iOS 7, it's concerning that iOS 8 appears to have stalled out. It's a demonstrably big update with a lot of useful, desirable features. What is it that's holding its adoption rate down?

Onerous OTA

If you want to know why iOS 6 and iOS 7 were picked up so much more quickly than iOS 5, look no further than the "over-the-air" (OTA) updater introduced in iOS 5. Previous updates needed to be installed through iTunes, which required hooking your phone or tablet to your computer, waiting for it to sync, and then downloading the update. iOS 5 made it so that most people would never need to physically connect their phones to their computers for anything.

The OTA updates for iOS 8 vary in size but tend to be just above a gigabyte, but the update requires upwards of 5GB of free space to actually install. This is an unusually large amount, and as Daring Fireball's John Gruber noted yesterday it's an especially hard pill to swallow for users of 8GB and 16GB (read: entry-level) iPhones and iPads. It's trivially easy to fill that space up with apps, music, videos, and pictures even if you're not an especially heavy user. Clearing out that kind of space when you don't have that much of it to begin with is a pain, especially when your phone or tablet seems to be working fine just the way it is.

The good news is that Apple does have an official solution for the problem. The bad news is that the solution involves hooking your phone or tablet up to iTunes to download the update, just like you did in the bad old days. An iTunes hookup is going to be even more out of the way these days than it was a few years ago, not least because Apple has spent the last three years coaching people to use their iDevices independently of their computers. This solution also doesn't help anyone who chooses to use an iPad as his or her primary computing device.

There's probably a perfectly reasonable technical explanation for why the iOS 8 update needs so much space to install, but there's not a reasonable explanation for why Apple continues to introduce new 8GB and 16GB iDevices as entry-level models in 2014. iOS is taking up more space on those devices all the time—according to our research, you can expect to lose around 740MB of usable space upgrading an iPhone from iOS 7 to iOS 8, and over 1GB on most iPads. Apple is running a business, and it wants to protect its margins, but bumping the storage capacities of the midrange and high-end iPhone 6 models while leaving the entry-level models at 16GB is a bad move. Maybe iOS 8's slow uptake is what will convince Apple to increase capacities across the line.

Leaving more devices behind

So far, new high-end iPhones typically outsell the phones they replace. This is reflected both in the ever-growing year-over-year sales numbers and the yearly Apple press releases about opening weekend sales. The flipside of this phenomenon is that every time a new version of iOS drops support for an older iPhone, Apple is leaving a larger number of devices behind.

It's difficult to get a feel for what kind of iPhones and iPads are still in active service at any given time, but take this data from Localytics as an example. As of early September, the iPhone 4 and older devices (i.e., those that can't upgrade to iOS 8 at all) account for about 18 percent of all iPhones in active use. The original iPad (dropped way back when iOS 6 was released) represents about 6 percent of all iPads in active use. Newer devices command much larger slices of both pies, but that's still a significant chunk of active devices that couldn't run iOS 8 even if they wanted to.

If we assume there's an iOS 9 released next year and that it drops support for older Apple A5-based devices like the iPhone 4S, iPad 2, and first-generation iPad Mini, there's an even larger number of devices that could be left behind. Remember, the iPhone 4 and 4S were the first ones to shed the iPhone's AT&T exclusivity and make their way to other carriers, so there are going to be many more of those phones out there than there are iPhone 3Gs or 3GSes.

This analysis isn't going to be perfect, because data like this is never 100 percent exact, and the introduction of the iPhone 6 and 6 Plus has already changed the charts substantially. Newer phones will shrink the share of older ones, and older devices will break or fail or be replaced and put in drawers. But as the number of iDevices out there rises, so too does the number of them that will be left behind when Apple drops them from the support list.

Slowdown begets slowdown

This next point is related to the last one—the Apple A5 chip powers a whole bunch of active iDevices. If Localytics' data is to be believed, the iPhone 4S represents around a fifth of all active iPhones. The iPad 2 and first-gen iPad Mini together represent nearly half of all active iPads, even more if you count the A5X in the third-generation iPad.

These devices all support iOS 8, but the problem is that none of them run it particularly well. We encountered noticeable slowdown on both our iPhone 4S and our iPad 2 compared to iOS 7.1.2 running on the same hardware. While we (and Mashable's Christina Warren, who also used iOS 8 on the 4S for several days) ultimately came to the conclusion that iOS 8's new features were worth the slowdown for most people, many of the other publications that reported our findings came to different conclusions. The message from Apple-centric publications like 9to5Mac, tech sites like Gizmodo and BGR, and even larger, more mainstream publications like Slate and The New York Times all picked up our reports and cautioned users either to think twice before upgrading or not upgrade at all.

Not everyone is going to read or listen to these reports, which is fine. The point is, A5 devices represent a big chunk of the iDevice population, and reports of slowdown spread widely enough that they may have dissuaded some from upgrading (or at least put them into "wait and see" mode). This problem may even be compounded by the size of the OTA update—if you've heard the update might make your phone slower and you need to go to some trouble to install it, maybe you'll just wait for a while to install it in the first place.

A buggy reputation

Speaking of widely disseminated reports, Apple caused a minor uproar soon after iOS 8's launch when version 8.0.1 completely broke cellular and TouchID support for many iPhone 6 and iPhone 6 Plus users. The actual number of devices affected was reportedly around 40,000 , a drop in the bucket compared to the ten-million-plus devices sold. If you're a company trying to sell people on an upgrade, you absolutely do not want "iOS 8" and "phone-breaking update" together in the same sentence barely a week after release.

Apple fixed the problem the very next day in iOS 8.0.2, which as far as I can tell has not been particularly buggy when judged next to any other early X.0 release of iOS (traveling for a week in Japan with an iPhone 3GS running iOS 4.0 was a uniquely terrible experience). But like the slowdown problem, this is one where perception is more important (and more damaging) than reality. Thanks to 8.0.1 and other, smaller reports of Wi-Fi problems, crashes, and Health and HealthKit oddities, iOS 8.0 has picked up a reputation for being buggy.

iOS 8.1, currently said to be launching later this month alongside new iPads and the Apple Pay service, will be Apple's first major opportunity to counter this perception. This is the first version of iOS 8 that can actually take real-world feedback into account, and Apple has already released two betas of the software in two weeks. There will always be edge cases, but hopefully the update smooths out the worst of the wrinkles.

We'll be keeping an eye on Apple's Developer Support graph to track iOS 8's growth—new data comes out every two weeks or so, and the next refresh will show us whether the last two weeks' slow growth was a bump in the road or a more serious problem. We're also planning a look at iOS 8.1 once the new version is released.