Chip Core# TDP 1-Core 1-Core All-Core All-Core
Power Freq Power Freq
3990X 64 280 W 10.4 W 4350 3.0 W 3450
3970X 32 280 W 13.0 W 4310 7.0 W 3810
3960X 24 280 W 13.5 W 4400 8.6 W 3950
3950X 16 105 W 18.3 W 4450 7.1 W 3885
As you can see, going from 4.35GHz down to 3.45GHz reduces power consumption by over 3x. Further, per-core power usage of the 3990x is very low overall.
This lower clock and lower per-core performance gives higher overall performance per watt.
Anandtech mentions this in their review
https://www.anandtech.com/show/15483/amd-threadripper-3990x-...
Chip Core# TDP 1-Core 1-Core All-Core All-Core Power Freq Power Freq 3990X 64 280 W 10.4 W 4350 3.0 W 3450 3970X 32 280 W 13.0 W 4310 7.0 W 3810 3960X 24 280 W 13.5 W 4400 8.6 W 3950 3950X 16 105 W 18.3 W 4450 7.1 W 3885
As you can see, going from 4.35GHz down to 3.45GHz reduces power consumption by over 3x. Further, per-core power usage of the 3990x is very low overall.
This lower clock and lower per-core performance gives higher overall performance per watt.