The iPhone 5’s overlooked killer feature

September 26, 2012, 10:37 AM UTC

The new iPhone 5. Best smartphone ever?

FORTUNE – By now, many are aware of that new record: 5 million iPhone 5s during its first week. For its part, Apple (AAPL) has done a good job marketing features like its meticulously machined thinner body and an even trimmer “Lightning” charging cord. But does it warrant all that early praise? We spent five days with it to parse through the hoopla and find out for ourselves.

Some may call the iPhone 5’s new design the latest example of evolution. That’s because upon first — and even second — glance, it could be easily mistaken for the iPhone 4 or 4S. The differences are in the details. It’s a sliver of a device that at 3.95 ounces, stands nearly 4.9 inches tall, 2.3 inches wide and .3 inches deep. That translates to being 18% thinner and 20% lighter when you hold it, but it’s the lightness you’ll notice immediately. On the back, much of that glass was swapped out for aluminum; two thin glass panes remain at the top and bottom.

The largest design difference is of course, that screen. It’s four inches instead of 3.5, the first for an iPhone, but certainly not the first for Android users already used to their devices swelling up to 4.5 — or in some hybrid-type devices — 5 inches. Viewed that way, the iPhone feels like it’s playing catch-up. But new owners and those upgrading will welcome the increased screen real estate. They’ll also appreciate the quality of the screen with better contrast.

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Behind the screen, there’s a new custom-made A6 processor, 1 GB of RAM, an improved 1.3 megapixel front-facing camera, and a tweaked 8 megapixel shooter that takes better dim shots. Apple was keen to point out at the unveiling the new A6 chip offers twice the speed of the previous model. For the most part, that speed boost is noticeable. Many apps, including the running app Nike+ (NKE), now load almost instantly, and switching between those apps flies. Twice as fast? We’re not so sure about that. But it’s extremely brisk.

Chalk a good chunk of those speed gains up to the integration of 4G Long Term Evolution (LTE) technology in the AT&T (T), Verizon (VZ), and Sprint Nextel (S) models. Our review unit was a white 64GB Verizon version, which yielded impressive results in San Francisco on its own and as a mobile hotspot. On average,, for instance, loaded in 4 seconds on Verizon LTE here in the city’s financial district; on AT&T 3G, it took 13 seconds. Obviously, results will vary from carrier to carrier and location to location.

Because Verizon launched its 4G network earliest, its nationwide coverage remains the most thorough, with 371 markets, compared to AT&T’s 65 and Sprint’s 18 or so. (For a helpful coverage map, reference here.) Call quality was very good on our unit —  voices were sharper and sounded fuller-sounding than previous iPhones — and we didn’t experience dropped calls.

This speed, in my opinion, is the iPhone 5’s killer feature. Much has been written about the incremental seeming nature of this generation’s improvements. And perhaps even more has been written about the quibbles (read on for some of my own). But most of the jaw-wagging about the new iPhone misses the point: Apple has made its iconic gadget radically faster in one single move.

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The real question is, how does battery life fare? Apple says to expect up to 8 hours on 3G or 4G and up to 10 hours with WiFi usage. The average user — one who checks email occasionally, listens to a few songs, and say, uses Maps for a few minutes through the day — can expect to all-day battery life. Those, like me, who are basically wedded to their phones, will still want to charge up later on or invest in an extended battery pack when available.

We won’t dive too deeply into iOS 6, since it’s a software update also available for the iPhone 4S and iPhone 4, though it’s worth noting the latter won’t officially offer  features like the 3-D “Flyover” feature in Apple Maps. But at the very least, it’s worth diving into Apple Maps itself. With iOS, Apple has basically swapped out Google’s mapping efforts for the company’s own. The switch has been met with criticism over maps of areas — mostly abroad — that lack detail or are simply inaccurate.

In our test runs, we found Apple Maps and its 3-D cityscapes quite pretty, but actually using the app proved somewhat problematic. Like Siri when it debuted last year, Maps this year doesn’t feel complete. When trying to walk to a nearby building, we were twice led in the opposite direction before the kinks seemed to work themselves out. At times, the phone had difficulty pinpointing my location, sometimes placing me three or four blocks away. Still, in other scenarios, Apple Maps performed fine, offering multiple routes to our destination. But the results were hit or miss, and it’s clear Apple still has a lot of work to do here.

A solid shot taken of downtown San Francisco with iOS6’s new Panorama mode.

A more minor issue concerned Panorama, a new software feature that essentially allows a user to take extremely wide shots. In practice, it worked largely as advertised (see above) yielding good results. Infrequently, we noticed some distortion around moving objects. And if you don’t “pan” the phone around to capture an image slowly enough, you risk some image tearing in corners.

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The use of a new Lightning connector cable may be an annoyance or a non-event. Want to use your old cords to charge the iPhone? You’ll pay $30 for an Apple adapter, or gamble with a cheaper third-party version. For me, it was a non-issue until last night, when I left the Lightning cable at work and realized my old ones wouldn’t cut it.

That’s not say the iPhone 5 isn’t in many ways a major improvement over iPhones past. It is, after all, faster and leaner. But you’ll want to triple-check those directions just in case.