Nvidia GPUs are made not to be used fully
If you actually fully load an nvidia GPU (such as via furmark) it will throttle really badly. The big issue here is that the voltage of the card is set not to drop below 0.712 (for the tuf 3090 oc) which is really bad when the power of the card needs to drop further to remain below the power limit. You end up with lowered frequency but the same voltage which is very far from ideal.
I was able to
replicate that issue with the tuf 3090 oc by using the "extreme burn-in" option. With 375W power-limit it was very far away from the 1600mhz i overclocked it to at 0.712v (dropping down to 1100mhz which is well below the stock base clock).
Some people have claimed that 0.712 would be some minimum for the architecture but that is extremely unlikely since at that voltage i was able to increase the frequency by 300mhz (20% overclock) showing that the voltage applies at stock is a lot higher than needed. Most likely this issue could have been resolved just by allowing lower minimum voltage (likely specified in the VBIOS). Unfortunately even when you extent the voltage curve in msi afterburner (picture above) it will not actually be followed by the card.
While this is a real issue it does not mean that nvidia GPUs are lame, the issue seems to have been that they had to go with the samsung 8 process which resulted in power-draw a lot higher than ideal even at low voltages (such as 0.718v).
You can still prevent down-clocking simply by increasing the power-budget a lot (if the VRM, PSU and cooler can handle it) but that might require modding the bios which will void the warranty (depending on what model you got). You would need over 550W of continuous power to support a fully loaded 3090ti at 0.712v & 1600mhz.
This also means that if you want to reduce the power-consumption of your card the better approach is simply to reduce the max voltage and accept that you will not have a stable power-consumption. That might also help to reduce the transient loads since the maximum height of the peaks will depend on the max voltage, this might still be worse if you are PSU/cooling constrained though. It also means the ability to increase the fps/W is rather limited.