my laptop has the core i7 8th gen H series processor and never exceeds 1.5 GHz on xtu when testing it. It is constantly power limit throttling for no reason, even when the temparatures are showing 50 degrees.
I have tried most of the fixes on youtube, it's does not seem to change anything.
Make sure your DELL bios is up-to-date. Make sure you are on windows 10 version 1903.
If that does not fix the problem, contact Dell support, as they are responsible for your support and Warranty.