https://www.macrumors.com/2018/07/17...ro-throttling/
Apparently the i9 runs so hot, and the MacBook Pro's cooling is so inadequate, that it can't maintain even base clock speed under a full CPU load (and forget about turbo boost, ever).
Last year's i7 MBP is faster because it doesn't run as hot.
But hey, it's super thin and quiet so that's all that matters, right?