https://www.gizmodo.com.au/2018/07/apple-promises-fix-for-throttled-macbook-pros-but-that-might-not-solve-everything/ :
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 applying the latest update from Apple, I re-ran the Geekbench build test on the i9. It’s slightly faster, but processor frequency stays stable when building (which is comforting).
— John Poole (@jfpoole) July 24, 2018
Still technically slower than the i7, but close enough that, practically, it doesn’t matter. pic.twitter.com/hOogCt6ZB0