Steam games underperforming on NixOS — GPU usage stuck at 60–70% (works fine on Arch/Windows)

System info: NixOS 26.05 & unstable channel
Desktop environment / window manager: Niri (Wayland) — also tested under GNOME (Wayland), issue persists
GPU AMD Radeon RX 6650 XT & CPU AMD Ryzen 5 5600X
The games I’ve tested: Counter-Strike 2(vulkan) Garry’s Mod
I have tried gamemode and switching to the Linux-Zen kernel,but it doesn’t work.
If anyone has experienced something similar or has suggestions on what to check or tune in NixOS. Let me know what logs or outputs would help. Thanks in advance!

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