Apple has issued a supplemental update to macOS High Sierra 10.13.6 that addresses the throttling issues exhibited by the 2018 MacBook Pro. Those who have installed the update report that they are seeing improved performance in the Core i9 model and that the CPU is now at least able to sustain the base clocks.
https://www.notebookcheck.net/Apple-issues-software-fix-to-address-thermal-throttling-in-new-MacBook-Pro.318232.0.html