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
I love watch how Apple plays with fire, their reputation and user's trust.
I just don't know how some of these notebooks can even pass the validation stage to market when they have such fundamental glaring flaws! It's a growing trend as CPU power is increasing and notebooks are getting thinner - running too hot & throttling way too much, not getting the 'advertised' performance. Way more emphasis needs to be placed on the engineering aspects of these notebooks rather than the cosmetics, as well as innovations in cooling technology - or just increasing the quality & manufacturing of the cooling solutions.