Our iMac review included a 3.06GHz Core 2 Duo chip inside, but we received the top-of-the-line iMac housing the more promising 2.8GHz Core i7 processor. Do more cores make up for lower clock speeds? Yes. Often 2X to 3X.
The Basic Differences in Chips
First off, I should note that the Core i7 chip has what Intel calls a "turbo mode." That is, when it's not utilizing all of its cores, it can dynamically overclock itself up to 3.4GHz on whatever single core is in use. It can, as shown in this video, work in steps. So you get the turbo benefit when using some of the four cores in this iMac's chip, but you also get it when all cores are being partially used. For example, if four cores are running but only at a fraction of their total capacity (less then 100%), the cores can use that electrical/thermal overhead to overclock to varying degrees. This should theoretically make up for the difference between the two-core 3.06GHz chip and the hyperthreaded quad core chip at a base of 2.8GHz.
The other thing to realize about these newer Core i7 chips are that they have no northbridge—or bus—between the memory and CPU. The memory controller is built right into the processor, and there's a new tech called QuickPath interconnect which connects the cores in a point-to-point architecture. Core i7 supports triple-channel memory (which would use three banks at once), but this iMac only came loaded with two banks of RAM filled. Like our other iMac, that's a 2GB + 2GB arrangement.
*Note that this machine also had a faster ATI Radeon 4850 video card with 512MB of RAM (versus the 4670 card in the other iMac) which may have impacted performance in several apps. I have no idea which of these apps uses the GPU to accelerate its tasks under Snow Leopard. (For example, Preview may use it to help render JPGs faster, or it may not. Apple could not tell me. In Adobe After Effects, the Radeon series of cards apparently is not supported for OpenCL acceleration. )
Performance with Multithreaded Apps
In short, any task we tried that expressly was written to either a) take advantage of multiple cores, or, b) take advantage of multiple cores through Snow Leopard's multicore middleware, Grand Central Dispatch, were 2 to 3 times faster. (More on that here.) These results include:
• 64-bit versions of Geekbench, which focus on CPU and memory tests.
• Adobe After Effects benchmarks
• Opening 20 images of Tokyo Tower that are 2000x2000 pixels and 35MB each.
Impressive stuff, but honestly, those tests were kind of uninteresting to me. I mean, those tests don't really have any correlation to my daily computing use. So on a whim, after benchmarking, I tested Handbrake, the DVD ripping software I love. It, too, was freaking fast.
I know the app is multithreaded, but I did not know what level of optimization it was written for. I was blown away by a 3x speed multiplier with the i7. On the Core i7 iMac, it took 43 minutes to rip a DVD, Storm Riders, a surfing film from the '70s featuring Gerry Lopez (my favorite) and others. On the Core 2 Duo machine, it took 147 minutes! I know this is basically a DVD read test coupled with decoding and video conversion, but the results have me excited because this is a real task that takes my computer a long time to do, performed by a program that hasn't been revised in a year.
Performance With Single-Core Optimized Apps (Otherwise Known as Reality)
Unfortunately, there are still very few applications that take advantage of multiple cores directly or via Snow Leopard's GCD, not even video-based, let alone general purpose computing.
Photoshop CS4 on the Mac, which is not set up to handle multicore processors, showed almost less than a 3% improvement using the Driver Heaven benchmark. Basic tasks, like booting and shutdown, saw virtually none. Playing the 1080p Quicktime trailer of Avatar consistently showed that the i7 was using 3% less of its total CPU than the Core2Duo, but I wonder if that's a result of the faster graphics card kicking in using CoreCL. Xbench, the old program that does a more comprehensive job of benchmarking a system from disks to processors, showed almost no difference.
I think Xbench, which hasn't been updated in years, is a solid benchmark for that old program that you depend on but has been long abandoned or at least ignored by its developer.
These scores, again, are in relation to the top line 3.06GHz Core 2 Duo iMac we tested. Some benchmarks have come in from the web comparing the i7 to the i5. Here's one that claims a 30% jump using Geekbench. Now we know Geekbench likes and does well with more cores and is a synthetic CPU test. But if the i5 is 30% slower, and the i7 pulls even with the 3.06 GHz Core 2 Duo chip in single threaded activity—most day to day activity—does that mean the i5 is slower than the cheaper Core 2 Duo? Maybe. Probably not 30%, since Geekbench is strictly CPU/memory and likes more cores, and this stuff does not translate so literally in the real world. But we can assume the i5 will have 30% less jump from the top tier Core 2 Duos, translating into a mere 1.3X to 2X speed increase from last gen chips on programs that like cores.
For the most part, in our review, I said that you should stick to the preconfigured options, upgrading to Apple's next recommended config before considering upgrades to the lower tier models. How does that advice change now that we've seen the i7? I don't know! I guess it depends if you're a betting man. If you think programs for Snow Leopard using GCD are coming, paying $200 to $500 bucks more from the top line Core 2 Duo chip for an i5 or i7 might make sense. The probability of you getting programs that can use those extra cores goes up if you are a graphics or video professional who expects to see support from Adobe, Apple, etc. (Apple already claims big jumps in Aperture that we weren't able to test.) Or if you rip a lot of DVDs! The rest of you? The Core 2 Duo stuff could be fine for today and fine for tomorrow. But the Core i7 is not worse for today and will definitely be faster tomorrow. It just costs more.
Me personally? I'd opt for the Core i7. I just might wait til the new iMacs refresh a bump and the i7 is cheaper and part of a standard build. But I'm patient like that.