Through a spokesperson, Apple explains:
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.
Earlier today, we noted the growing controversy surrounding so-called “thermal throttling” that’s been affecting some 2018 MacBook Pro users. This stemmed from a YouTube video by David Lee, in which he complained that his new top-of-the-line model with the 2.9GHz 6-core 8th-generation Intel Core i9 processor would begin throttling after just a “few seconds” of high-intensity work.
Instead of clocking at 2.9GHz, Lee’s MacBook Pro only registered a 2.2GHz. Soon after sites such as AppleInsider reported similar findings.
Announcing today’s release of a fix, Apple concludes:
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.
You can download the bug fix here or through the Software Update mechanism on your Mac.