Apple identifies bug behind MacBook Pro speed throttling, offers quick fix

Apple has been catching heat over the performance of its new MacBook Pro laptops, but the company hopes a new software update will cool off outraged power users. 

Following reports of heat issues leading to throttled performance, Apple has determined that a simple software bug was to blame, and has released what it hopes is a quick fix.

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Apple promises to fix MacBook Pro CPU throttling bug


“Following extensive performance testing under numerous workloads, we’ve identified that there is a missing digital key in the firmware that impacts the thermal management system and could drive clock speeds down under heavy thermal loads on the new MacBook Pro,” an Apple spokesperson tells Techhnews. “A bug fix is included in today’s MacOS High Sierra 10.13.6 Supplemental Update and is recommended.

“We apologize to any customer who has experienced less than optimal performance on their new systems. Customers can expect the new 15-inch MacBook Pro to be up to 70 percent faster, and the 13-inch MacBook Pro with Touch Bar to be up to 2x faster, as shown in the performance results on our website.”

The MacOS 10.13.6 update is available here. 

The surprise midsummer update to the 13- and 15-inch MacBook Pro line, announced July 12, includes new eighth-gen Intel CPUs, up to the high-end, six-core Core i9 in the 15-inch MacBook Pro. Also new are bigger solid-state storage drives, an updated T2 security chip and a True Tone screen that adjusts color temperature based on the available light.

Heat throttling during intense workloads

But the biggest buzz about the new MacBooks came from some early adopters who claimed the premium Core i9 versions, which cost a minimum of $2,799, were prone to heat-related throttling. That means the laptops dropped their CPU clock speed when internal temperatures spiked, leading to slower performance than older models with less-advanced Core i7 processors.

YouTube tech personality Dave Lee first brought the issue to public attention with a video in which he demonstrated the heat and throttling issues. These results were soon replicated by others, including a demo session from Leo Laporte on The New Screen Savers.  

The problems only became evident when running long, intense processes, such as encoding longer 4K videos. Here in the Techhnews Labs, where we’re testing a $4,699, 15-inch, Core i9 configuration of the new MacBook Pro, the new model showed performance improvements over last year’s Core i7 MacBook in our initial tests, which were short enough so as not to trigger the heat-related throttling.

When setting up an intensive workload to deliberately push the system, we were able to easily cause the Core i9 CPU in our 2018 MacBook Pro to ping-pong its internal temperature and CPU clock speed up and down rapidly, in both a video encoding and 3D gaming test. 

In comparison, the 2017 15-inch MacBook Pro maintained a much more even temperature and CPU clock speed while performing the same tasks.


On the left, a 2017 Core i7 MacBook Pro runs a video encoding test. On the right, a 2018 Core i9 MBP sees its internal temperature and CPU clock speed throttle up and down wildly while performing the same task. 

Dan Ackerman/Techhnews

So, while not all users would see their performance compromised by the heat throttling issues, the sort of power users who would be rendering long-form, high-res video would. Those are the pro users to whom Apple was targeting this round of updates. And suffice it to say, no one paying up to $6,699 for a top-line configuration wants to find out that it can’t perform as well as an older version on the most intense tasks. (Imagine buying a Ferrari, only to be told you can’t exceed 60 mph for more than 5 minutes at a time.)

Apple’s fix is already here

The initial fear was that Apple had effectively crammed a desktop-class processor into a thin laptop that didn’t have adequate heat management. Indeed, the chassis of the 2018 model is essentially unchanged from its 2017 predecessor, both from the outside and based on teardowns that have appeared since its retail availability earlier this month.

But Apple maintains that the culprit is software-related. The company says it has identified a previously unknown bug that affected the system’s thermal management system. Under some workloads, this could drive the CPU to throttle itself more than necessary, leading to the unexpectedly choppy performance.

The issue affects all of Apple’s new MacBook Pro models, both 13- and 15-inch. The entry level 13-inch MacBook Pro without the Touch Bar isn’t affected, as it has not been updated this year.

The software update is available to download now, and will eventually be preloaded on new MacBook Pros sold at retail stores. Until that happens, new purchasers will have to apply the update themselves.

Techhnews Labs will continue to test the new MacBook Pro with Apple’s software patch and will include updated performance results in our full review.

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