Conclusion: What Just Happened?
Power Consumption
If you thought Kaby Lake-X looked pretty ordinary so far, hold on to your thermal paste. For no added performance (in fact, less performance more often than not), the Kaby Lake-X CPUs consumed between 15 and 20% more power under load in our Excel test. Perhaps worse still, X299 platform can be seen using almost 20% more power at idle.
We have even more bad news for would-be Kaby Lake-X buyers. Above we see that during extended periods of load the 7740X and 7640X consumed over 40% more power! WTF?
What Just Happened?
Going into this review, we suspected Kaby Lake-X wouldn't be anything too impressive, mostly because these quad-core CPUs have no place on a high-end desktop platform that they can't fully drive. The fact that they provide weaker performance and greater power consumption than the original Kaby Lake processors only adds insult to injury.
These performance issues could be blamed on the X299 board we used and motherboard manufacturers sure appear to be somewhat of a scapegoat for this sloppy release. That said, the Asrock X299 Taichi appeared flawless for me when testing the higher-end 7800X, 7820X and 7900X CPUs.
After recording such odd results I looked around to see what other reviewers found during their testing. Mixed results are the norm with Kaby Lake-X being slower for the most part across a range of hardware. However, the confusing power consumption figures I recorded haven't been seen by everyone.
For instance, ProClockers found the 7740X to consume 15% less power than the 7700K when comparing total system consumption and they used the Asus Prime X299-Deluxe motherboard for those wondering. Meanwhile, LAN OC found the 7700K and 7740X to consume the same amount of power under load, though at idle the 7740X was 16% hungrier, and they used the Asus Prime X299 Deluxe as well, so those two reviews alone are quite conflicted.
I'd also like to mention Vortez's review, which found similar numbers to me. Under load, their 7740X system ate up 35% more power than the 7700K and they used the Gigabyte Aorus X299-Gaming 3.
Whether we blame the motherboard makers, the rushed platform in general or perhaps even the different testing methods used by us reviewers, we all seem to agree that Kaby Lake-X is overall slower than Kaby Lake, and that's obviously not good news.
The Core i7-7740X and Core i5-7640X make absolutely no sense to me. I can't think of one good reason why they exist or what Intel's plan might have been here. To pull a quote from my Intel Core X vs. AMD Threadripper article published at the start of Computex:
Perhaps even more puzzling is the $240 Core i5-7740X which is another quad-core part, though it lacks Hyper-Threading support, so just 4 threads are on offer here. For $20 more than the Ryzen 5 1600, Intel is giving us a traditional Core i5. I won't bother reading out the specs, I'm pretty sure this is going to be another Core i3-7350K situation where I simply tell you guys, don't buy it.
Here we are five weeks later and I'm confident in suggesting that you shouldn't buy a Kaby Lake-X CPU. Glad to have that off my chest.
score
Pros: Kaby Lake-X offers no apparent advantages over Kaby Lake and actually performs worse in every category.
Cons: Slightly slower than Kaby Lake yet consumes considerably more power and costs more courtesy of the X299 motherboard.