the bios is not wokring correctly and it recognize in first place charge over TB3 instead of directly connected power.
The only solution, when my CPU is throttled, I found is to unplug the notebook AC adaptor for a while.. then the TB3 is re-connected [using external screen,kebyoard, headset, so I see all going dark for few seconds], then while notebook runs on battery and recognized again the TB3 [which have it's external power] - I will plug in the original Dell AC adaptor into notebook and all is good.
This is a problem when I start notebook with TB3 connected at boot time.. once I do this disconnect / reconnect AC - just once after the boot, all is then fine.. no throttling due to unsufficient power or due to charging the battery.
I do believe this just need small BIOS update which will always look for directly connected power adaptor in first place, and power over TB as secondary option.
anyone reading this if you have any power how to flag this to Dell support to trigger BIOS fix I would be thankful.