On the surface, the iPhone 3GS doesn't look much different to the older 3G, especially if the latter is running the latest 3.0 version of the firmware (more about that later). But one of the key changes isn't a new feature, it's the 3GS' ability to do exactly what the old iPhone can do, just faster.
Apple is still pretty cagey when it comes to the exact hardware specification in the iPhone, but the 3GS has seen an increase in speed for the main processor, along with a new graphics chip, while there's also more memory on tap. The result - according to Apple at least - is that the 3GS is up to twice as fast as the 3G, which is a pretty bold claim.
In practice though, I can assure you that Apple is not exaggerating - the 3GS is significantly quicker than the 3G in pretty much every area. My guess would be that the new graphics chip plays the biggest role in the 3GS' ode to speed. There's no doubt that the 3GS renders graphics far, far faster than the 3G, whether that be on a web page or even in the phone's own user interface.
Mobile Safari has always been one of the iPhone's trump cards, but the 3GS takes it to a new level of usability, rendering graphics heavy web pages in the blink of an eye. In fact, in this respect the 3GS really does live up to Apple's claims of it being twice as fast as the 3G. Connecting the 3GS and my iPhone 3G up to the TrustedReviews Wi-Fi network, I then opened www.trustedreviews.com on both handsets simultaneously. The 3GS simply blew the 3G away in terms of speed, with the page rendered in less than half the time.
Well, that's what happened when I ran that test yesterday. Today things played out very differently indeed. Today I tried exactly the same page load test and the 3GS was far, far slower than the 3G. This had me very confused until I ran a data speed test and found that while my iPhone 3G was managing around 700kbps on the office Wi-Fi connection, the 3GS was reporting download speeds of between 19 and 40kbps. So clearly the problem was to do with Wi-Fi rather than the speed of page rendering.
A bit of digging around the Internet showed that I'm not the only one who's had Wi-Fi issues with the 3GS, with most reports indicating unreliable or intermittent performance. This turned out to be the case, since when I tried the same test an hour later the 3GS once again spanked the 3G, and running the speed test again turned in a download speed of over 600kbps.
Taking Wi-Fi out of the equation though, the 3GS is consistently faster, whether you're switching between apps, or simply opening up the settings page. On graphic heavy apps the new graphics chip will be making a difference, but also the extra memory no doubt keeps the 3GS nipping zipping along.
Another area where the 3GS has a potential speed increase over its predecessor is 3G data. The 3GS supports 7.2mbps HSDPA, which is currently the fastest cellular data standard available. However, the iPhone 3GS supporting 7.2mbps HSDPA is only part of the equation, since even if a network provider offers full 7.2mbps HSDPA, most of the time you'll only see a fraction of that.
Running the iPhone Speed Test again, I compared the 3GS to an iPhone 3G on the T-Mobile network instead of O2. The result was the 3GS on O2 turning in a very respectable 710kbps, while the 3G on T-Mobile managed more than double that at over 1.6mbps! So, in some respects the faster 3G capability of the iPhone 3GS is moot, since it would appear that O2's 3G network can't really make the most of it.
I did the same test with an iPhone 3G on O2, and its download speed was pretty much identical to the 3GS. However, once again the 3GS' ability to render graphic heavy web pages quickly meant that it could still load, say, TrustedReviews faster than both 3G handsets, regardless of network. On the plus side, anyone who buys an iPhone 3GS should be safe in the knowledge that it will just get faster as O2 improves its 3G network.