https://www.cnet.com/news/apple-macbook-pro-speed-throttling-patch-tested/

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. In our initial testing, the MacOS update appears to correct the up-and-down CPU throttling, and we're continuing to run additional tests.  

"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 CNET. "A bug fix is included in today's MacOS High Sierra 10.13.6 Supplemental Update and is recommended.