Apple says that thermal throttling issues with the 2018 MacBook Pros discovered over the past week are due to a software flaw in the system's thermal management software. A free software update supposedly fixes the problem on all the new 13-inch and 15-inch models, not just the models boasting an Intel Core i9 chip.
An Apple spokesperson had this to say to MacRumors:
"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."