SUMMARY
kwin_wayland defaults to llvmpipe (software rendering) on Intel 11th Gen Iris Xe, causing very high CPU usage; MESA_LOADER_DRIVER_OVERRIDE=iris fixes it.

STEPS TO REPRODUCE
- Use a system with an Intel 11th Generation CPU (i7-1165G7 in my case) with integrated Iris Xe graphics.
- Install Kubuntu 25.04
- Log into a Plasma Wayland session.

OBSERVED RESULT
The kwin_wayland process consumes a very high percentage of CPU (80-90%) even when the desktop is idle, causing the basically the entire system to lag, including the cursor.
intel_gpu_top shows minimal (1-2%) usage on the "Render/3D" engine of the integrated GPU
perf report for the kwin_wayland process shows that the majority of CPU time is spent in llvmpipe and libgallium functions, so it's doing software rendering.

EXPECTED RESULT
kwin_wayland should utilize the Intel Iris Xe GPU for hardware-accelerated rendering.
CPU usage for kwin_wayland should be low when the desktop is idle.
intel_gpu_top should show appropriate GPU level for desktop compositing.

SOFTWARE/OS VERSIONS
Linux/KDE Plasma: Kubuntu 25.04
KDE Plasma Version: 6.3.4
KDE Frameworks Version: 6.12.0
Qt Version: 6.8.3

ADDITIONAL INFORMATION
CPU: Intel 11th Generation Core i7-1165G7 (with Iris Xe Graphics)
The X11 session works correctly with hardware acceleration and normal CPU usage.

The issue is resolved by setting the environment variable MESA_LOADER_DRIVER_OVERRIDE=iris for the Plasma Wayland session (by creating ~/.config/plasma-workspace/env/90-mesa_iris_override.sh with export MESA_LOADER_DRIVER_OVERRIDE=iris and making it executable).

Initial perf report:
"Samples: 937K of event 'cycles:P', Event count (approx.): 915207451975
  Children      Self  Command          Shared Object                  Symbol
+   12.52%     0.00%  llvmpipe-0       libc.so.6                      [.] __GI___clone3                                                                                                                    
+   12.52%     0.00%  llvmpipe-0       libc.so.6                      [.] start_thread                                                                                                                     
+   12.52%     0.00%  llvmpipe-0       libgallium-25.0.3-1ubuntu2.so  [.] 0x000078d4c57cfb9c                                                                                                               +   12.51%     0.00%  llvmpipe-0       libgallium-25.0.3-1ubuntu2.so  [.] 0x000078d4c5b9431f"                                                                                                                                                                                                   

dmesg output (related to i915 or drm) shows the kernel driver loading, but Wayland session still defaulted to llvmpipe