How to reproduce:
1. Extract and open the attached “IN-112634.zip“ project
2. Go to Assets → Scenes and open the “Terrain.prefab“
3. Under “Layers“, select any of the “Layer Palette” textures (Do not select the check box, only highlight the texture)
4. Try drawing the texture on the “Terrain” GameObject
5. Observe the Editor

Expected result: The Unity Editor maintains smooth performance
Actual result: Performance is significantly degraded; the Editor’s FPS drops considerably in the profiler

Reproducible with: 1.1.2-preview (2023.1.0a1, 6000.0.57f1, 6000.2.4f1, 6000.3.0b1), 5.3.0 (2023.1.0a1, 6000.0.57f1, 6000.2.4f1, 6000.3.0b1)

Reproducible on: Windows 11
Not reproducible on: No other environment tested

Notes:
- DirectX12 exaggerates the issue; performance seems better on DirectX11
- It appears to be related to GPU performance, with the issue being more pronounced on Intel(R) UHD Graphics compared to the dedicated Nvidia 3500 GPU.