https://motherboard.vice.com/en_us/article/kzyz4x/apple-says-its-going-to-fix-the-macbook-pro-throttling-problem

Thin and light laptops are great, but can cause a host of performance problems. Last week, Apple released the MacBook Pro 15, which included a model with a powerful Intel i9 chip that it marketed to professionals. One problem—the i9 runs hot inside the little laptop and the system throttles down its speed to keep the system from overheating.

YouTuber David Lee was the first to discover and publicize the problem via his YouTube page, but other customers with i9 MacBook’s found the same problem. Lots of people with i9s experienced throttling during intense computations such as rendering a video—the kinds of jobs you buy a powerful CPU like an Intel i9 for.