https://arstechnica.com/gadgets/2018/07/we-tested-throttling-on-the-macbook-pro-now-apple-says-it-has-a-firmware-fix/

Today, Apple will release macOS High Sierra 10.13.6 Supplemental Update—an update the company says will fix the widely reported performance throttling problem with the new, 2018 MacBook Pro.

Since the device's launch about a week ago, some users reported that sustained, heavy utilization of both the CPU and the GPU caused the 15-inch MacBook Pro's Intel Core i9 processor to drop below its base clock frequency, negatively impacting performance for some demanding workloads.