Apple has issued a statement about the thermal throttling users have seen with the new MacBook Pros:
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.
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% 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.
After YouTuber Dave Lee posted his video showing his new Core i9 MacBook Pro slowing to a crawl while exporting video from Adobe Premiere, as well as reports that the processor on the new MacBook Pros was being throttled all the way down to 800MHz, the company launched an investigation (and contacted Lee for more information about his specific project and settings). Apple’s own internal performance testing hadn’t triggered the issue, which turns out to not be app specific—Premiere, you’re out of the penalty box—and tends to affect heavy workloads that take place over an extended period of time.
I find it worrying that Apple’s internal performance testing didn’t run into this, but I’m glad that it seems to be fixable via software. I bet Jason’s tweet to me explains the discrepancy:
re: "I find it worrying" – my best guess is that they tested on devices that didn't lack that key, because they were internal devices rather than final firmware from the factory
— Jason Snell (@jsnell) July 24, 2018