When Intel announced its 14nm Core M processor it declared that this would be the chip that eliminated consumer perceptions of an x86 “tax” once and for all.* Broadwell, it was said, would bring big-core x86 performance down to the same fanless, thin-and-light form factors that Android tablets used, while simultaneously offering performance no Android tablet could match. It was puzzling, then, to observe that some of the first Core M-equipped laptops, including Lenovo’s Yoga 3 Pro, didn’t review well and were dinged for being pokey to downright sluggish in some cases.

A new report from Anandtech delves into why this is, and comes away with some sobering conclusions. Ever since Intel built Turbo Mode into its processors, enthusiasts have known that “Turbo” speeds were best-case estimates, not guarantees. If you think about it, the entire concept of Turbo Mode was a brilliant marketing move. Instead of absolutely guaranteeing that a chip will reach a certain speed at a given temperature or power consumption level, simply establish that frequency range as a “maybe” and push the issue off on OEMs or enthusiasts to deal with. It helped a great deal that Intel set its initial clocks quite conservatively. Everyone got used to Turbo Mode effectively functioning as the top-end frequency, with the understanding that frequency stair-stepped down somewhat as the number of threads increased.

Despite these qualifying factors, users have generally been able to expect that a CPU in a Dell laptop will perform identically to that same CPU in an HP laptop. These assumptions aren’t trivial — they’re actually critical to reviewing hardware and to buying it.

The Core M offered OEMs more flexibility in building laptops than ever before, including the ability to detect the skin temperature of the SoC and adjust performance accordingly. But those tradeoffs have created distinctly different performance profiles for devices that should be nearly identical to one another. In many tests, the Intel Core M 5Y10 — a chip with an 800MHz base frequency and a 2GHz top clock — is faster than a Core M 5Y71 with a base frequency of 1.2GHz and a max turbo frequency of 2.9GHz. In several cases, the gaps in both CPU and GPU workloads are quite significant — and favor the slower processor.

While this issue is firmly in the hands of OEMs and doesn’t reflect a problem with Core M as such, it definitely complicates the CPU buying process. The gap between two different laptops configured with a Core M 5Y71 reached as high as 12%, but the gap between the 5Y10 and the 5Y71 was as high as 36% in DOTA 2. The first figure is larger than we like, while the second is ridiculous.

None of this means that the Core M is a bad processor as such. But it’s clear that its operation and suitability for any given task is far more temperamental than has historically been the case. Even a 12% difference between two different OEMs is high for our taste — if you can’t trust that the CPU you buy is the same as the core you’d get from a different manufacturer, you can’t trust much about the system.

Is this an opportunity for AMD’s Carrizo?

Officially, AMD’s Carrizo and Intel’s Core M shouldn’t end up fighting over the same space; the Core M is intended for systems that draw up to 6W of power, and Carrizo’s lowest known power envelope is a 12W TDP. That doesn’t mean, however, that AMD can’t wring some marketing and PR margins out of the Core M’s OEM-dependent performance.

When AMD talked about Carrizo at ISSCC, it didn’t just emphasize new features like skin-temperature monitoring, it also discussed how each chip would use Adaptive Voltage and Frequency Scaling, as opposed to Dynamic Voltage and Frequency Scaling. AVFS allows for much finer-grained power management across the entire die — it requires incorporating more control and logic circuitry, but it can give better power savings and higher frequency headroom as a result.

If AVFS offers OEMs more consistent performance and better characteristics (albeit in a higher overall power envelope), AMD may have a marketing opportunity to work with — assuming, of course, that it can ship Carrizo in the near future and that the chip is competitive in lower power bands to start with. While that’s a tall order, it’s not quite as tall as it might seem — AMD’s Kaveri competed more effectively against Intel at lower power than in higher-power desktop form factors.

Leaving AMD out of the picture, having seen both the Core M and the new Core i5-based Broadwells, I’d have to take a newer Core i5, hands down. Core M may allow for an unprecedented level of thinness, but the loss of ports, performance, and battery life doesn’t outweigh the achievement of stuffing an x86 core into a form factor this small — at least, not for me. Feel differently? Sound off below.