Heyo!
I am trying to use CachyOS with a laptop and eGPU but I’m encountering an issue where the external screen’s performance is ridiculously bad when it’s connected to the eGPU directly, but perfectly fine on the laptop’s internal screen. Stranger than that, the amount the performance drops seems to be related to how much of the displayed image is moving.
I’ve already been trying to find a solution for a day or two but no luck so far.

My setup:

• OS: Fresh install of CachyOS / KDE (installed with the dock connected)
• Laptop: GPD Pocket 4 (with USB4, 40 Gbps port)
• eGPU Dock: Peladin Link-S1 (Thunderbolt 3, 40 Gbps)
• GPU: RTX 4060 TI (also tested with a 2080 TI)
• eGPU cable: 0.5m USB4 40 Gbps cable (also tested with a 0.8m Thunderbolt 4 cable)
• External monitor: MSI MAG27CQ 16:9 1440p 144hz (120hz for HDMI)

Issue details:

• When the game window is displayed on either the laptop’s internal screen or an external screen connected to the laptop’s internal HDMI port it runs great at up to ~120+ FPS.
• When the game window is displayed on the external screen connected to the GPU directly it goes down to ~20 FPS, depending on how much of the game window is displayed on the external screen.
• Using the 2080 TI had similar performance, except the external monitor framerate only went down to ~40 FPS
• For comparison, using the laptop’s iGPU with no eGPU connected it gets me around ~50-60 FPS.
• These tests I’ve all done on low settings to maximize the frame rate. I briefly tested with high settings as well, the best case framerates got expectedly reduced but the worst case framerate remained unchanged. Eround 20 FPS in either case.
• These frame rate issues while not as easily measurable also happen with videos or normal desktop use. In fact any moving image on the external screen connected to the GPU tanks the frame rate, like videos, moving around windows or even just moving a slightly enlarged mouse cursor.

I also have a brief video demonstrating this issue, but I’m not sure how I’d best submit that here since it doesn’t allow video files.

I was FINALLY able to fix this myself by using all-ways-egpu, which at first didn’t work at all but it turned out to be entirely of my own making. In my attempts to get it working I had manually set an environment variable for __EGL_VENDOR_LIBRARY_FILENAMES=/usr/share/glvnd/egl_vendor.d/50_mesa.json but as it turns out, it needs to be set conditionally and CachyOS already does it automatically in /etc/profile.d/nvidia-rtd3-workaround.sh and my manual one broke that. So I deleted the variable that I had added and it started working finally!

Shoutout to this issue which helped me figure it out.

Anyways, here’s a lesson learned: If you configure something in an attempt to get something working, and it does not make it work, always undo whatever changes you’ve made. Chances are whatever you just did (which evidently wasn’t working anyways) may end up being the cause of some other breakage later down the line.