ConclusionThe idea behind this review is to show percentage gains from increased clock speeds of the Core i7-7700K and its predecessor, the i7-6700K, on the GeForce GTX 1080 and the Radeon RX 480. The Core i7-7700K "Kaby Lake" is a very fast processor on its own. I'm sure it will be able to drive some of the most powerful 4K Ultra HD gaming rigs in the months to come. Compared to its immediate predecessor, however, it's safe to conclude that it is a disappointment. The chip isn't noticeable faster than the Core i7-6700K in any of our tests, which are focused on actual game performance - not complex CPU calculations.
At least the Core i7-3770K "Ivy Bridge" gave users the benefit of a new process (22 nm at the time, over the 32 nm of "Sandy Bridge"), but the i7-7700K can't even offer something like that, at least not to PC enthusiasts and gamers. While it offers a significant overhead in maximum overclocking, we did have some difficulty getting the i7-7700K to run at 5.00 GHz game bench-stable on air cooling, especially Prime95's AVX testing proved difficult. Increasing voltage far enough to make it stable would have the CPU running into its temperature limit quickly. Remember, this is on air cooling; on water, getting 5 GHz should be much easier. We settled for 4.90 GHz, which took quite a bit of work, fine-tuning the balance between voltage and heat. Nevertheless, assuming you are buying new, and assuming the Core i7-7700K is priced similarly to the 6700K, I see no reason not to buy the new CPU, which should give you another 500 MHz in CPU clock, which, however, doesn't translate into any significant gaming performance gains, as this article shows.
We initially planned to get this review done with just two GPUs, the high-end GeForce GTX 1080 and the performance-segment Radeon RX 480. When processing our results, we noticed an interesting trend as the RX 480 was posting bigger gains due to increased CPU speed than the GTX 1080. This phenomenon is more visible in the lower 1080p resolution, rather than in 1440p or 4K (where the RX 480 doesn't provide playable frame-rates in most games anyway). We initially dismissed this as behavior a performance-segment GPU designed for lower resolutions would exhibit. We then pulled out a GeForce GTX 1060 6 GB, a comparable graphics card from NVIDIA to the RX 480 and noticed that such gains were not noticeable.
This has us conclude that the RX 480 has a slightly higher CPU overhead than the two NVIDIA cards. Graphics drivers need the CPU to crunch some serial processing loads at various stages of the graphics rendering pipeline, before they get fed to the GPU. Previously, AMD's driver overhead was a vague assumption backed by very few benchmarks, but this article is conclusive evidence that the AMD graphics ecosystem definitely has more CPU overhead than NVIDIA's.
Looking at the bigger picture, clock-for-clock, there are close to zero IPC gains between the i7-7700K and the i7-6700K. At 4.50 GHz, both chips showed nearly identical performance with both the GTX 1080 and the RX 480. Comparing the two chips at stock speeds, the i7-7700K is faster by as much as 6%. This is most probably because the chip is clocked just so much higher out of the box (4.20 GHz core with 4.50 GHz Turbo as opposed to 4.00 GHz core with 4.20 GHz Turbo). 4.90 GHz is still a decent 20% OC we achieved compared to the 4.20 GHz core.
There are games in which the GeForce GTX 1080 benefits by as much as 5% from an i7-7700K overclocked to 4.90 GHz. In "Mafia III", you get a performance gain of 5.8% at 4K resolution. These gains are slightly higher at lower resolutions, where games tend to be more CPU limited because they are running at higher framerates, with each individual frame having similar overhead. Given these performance figures, it's hard to recommend an upgrade to the Core i7-7700K from the i7-6700K. There are no IPC gains to be had, overclocking isn't that much better, and today's graphics cards don't significantly benefit from the faster processor. If you want to spend upgrade money for gaming, the better investment is a faster graphics card. However, if you are buying new, upgrading from something older than Haswell, there is no reason to buy the i7-6700K unless you can find it used at a significant discount.