RTX 4080 + Radeon W600

[billdigo]

I'm using two cards in my system: RTX 4080 (connected to the main monitor) + Radeon W600 (6 outputs).
In Output Advanced, when I add outputs from the RTX4080 everything is fine, the FPS is high.
But if I add a screen using a single output from the W600, the FPS drops a lot and the processing of the W600 goes to 99% usage.
Isn't processing supposed to stay on the RTX 4080?
Why does the FPS drop so much when I use a W600 output if it is not connected to the main monitor?

Thanks in advance for your help

[Zoltán]

The processing stays on the renderer GPU, but that means the texture it rendered has to be transferred to the GPU with the output. That will come with a performance hit. So the best performance woudl come from a single GPU and external display splitters like Datapath FX4s

A lot depends on the drivers, and using two different GPU manufacturers is also probably not the safest way to go at it.

Does the CPU have enough PCI-e lanes to handle the GPUs?