Operating system: Windows 10
Graphics card: GTX1080Ti
Broken: 2.8 latest master
Short description of error
When baking textures with Cycles, increasing tile size significantly decreases texture bake rendering time without any impact on resulting baked texture quality. This signifies extreme inefficiency of texture baking in Cycles with default setting of tile size at 32.
Exact steps for others to reproduce the error
1, Make sure you have nVidia GPU enabled as CUDA device in Preferences
2, Grab the scene here: https://drive.google.com/open?id=1fvrpnFcg4vClbmPdNwncw3nR9GeoLfJb
3, Open the scene, make sure BakeTest Image Texture is selected in the node view, and click the Bake button in Cycles render settings
4, Observe the long bake rendering time (2:04 minutes in my case)
5, Once bake is done, save the baked BakeTest image from the Image View editor for comparison later.
6, In Cycles render settings panel, increase Tile Size parameters from 32x32 to 1024x1024 (The size of the baked texture)
7, Click the bake button again and observe the short rendertime (0:08 minutes in my case)
8, Once bake is done, save the BakeTest image again for comparison with previous one.
9, Compare the both saved images.
Result: With tile size set to default 32x32, baking of the texture takes 15.5x longer than with tile size set to resolution of the baked texture, yet there is no difference in the baked texture quality which would justify almost 16x longer rendertime.
Expected: Cycles will always bake textures at optimal performance, regardless of tile size.