Bug 496941

SUMMARY
Hello!
I have a laptop with an Nvidia GPU, and an AMD iGPU. The setup is as follows: for the internal display, and HDMI port, the Nvidia GPU's output is passed through the iGPU, and when using a USB-C DisplayPort, the output is directly connected to the Nvidia GPU.

When connecting to the DisplayPort, to the dGPU, the performance of KWin degrades severely, and can't maintain a smooth refresh rate (I know the performance overlay isn't supposed to be used for debugging; it reports a quite low ~40fps though, which feels about right for what I'm seeing). The monitor connected in this case is a 1440p 165Hz display. HDR and VRR do not make any difference.

Connecting this display through the HDMI port, which passes through the iGPU, everything fine. Higher resolution displays also do work smoothly when they are connected to the HDMI port. The only difference I found in that case, is that the kwin_wayland process uses the iGPU for compositing, and when connected directly to the dGPU, it uses that Nvidia card for compositing.

I'm not sure how to go about troubleshooting this issue, if anything is needed from the system, do ask. Thanks!

STEPS TO REPRODUCE
1. Have a laptop with an AMD iGPU and Nvidia dGPU
2. Connect directly to Nvidia dGPU
3. Observe

OBSERVED RESULT
Performance of the compositor is degraded when rendered with the dGPU.

EXPECTED RESULT
Performance of the compositor is on par with, or better than the iGPU, when using the more powerful dGPU.

SOFTWARE/OS VERSIONS
Operating System: Arch Linux 
KDE Plasma Version: 6.2.4
KDE Frameworks Version: 6.8.0
Qt Version: 6.8.0
Kernel Version: 6.12.1-arch1-1 (64-bit)
Graphics Platform: Wayland
Processors: 16 × AMD Ryzen 7 5800HS with Radeon Graphics
Memory: 15.0 GiB of RAM
Graphics Processor: AMD Radeon Graphics

ADDITIONAL INFORMATION
Nvidia driver version: 565.57.01
dGPU: NVIDIA GeForce RTX 3050 Laptop GPU