Falling is normal in this case.
The GeForce RTX 4090 is an incredibly powerful graphics card. However, in one of the use cases, you need to prepare for the fact that it will lose more than 20% of its performance.
The first experiment with the RTX 4090 as an external graphics card showed just such a decrease. One user connected an RTX 4090 to an Asus Zenbook 14X Space Edition laptop via a Razer Core X chassis.
Tests have shown that in this mode, the Nvidia flagship is 8-28% slower than if it is installed in a regular PC. But the bottom line is that such a decrease was recorded for a well overclocked video card (+249 MHz for the core, + 1500 MHz for the memory and a power limit of 133%). That is, without overclocking, the drop will be even greater.
If we subtract about 20% of the performance from the RTX 4090, we get exactly the result of the RTX 4080. Of course, even here we see that everything depends on the test, and there are no results in games yet, but in any case, you need to understand that you still need to reach the level RTX 4090 in its normal form when creating an external video card will not work.