Juraj, creates a plane with teapot + CoronaSun+Sky, with all settings by default for everything:
Teapot shader n° 1
(*) CoronaMtl
(*) Diffuse = Bitmap
5.2M Rays/s
Teapot shader n° 2
(*) CoronaMtl
(*) Diffuse = CoronaUvw + Bitmap
4.0M Rays/s
Teapot shader n° 3a
(*) CoronaMtl
(*) Diffuse = CoronaUvw + CoronaUvw + Bitmap
2.1M Rays/s
Teapot shader n° 3b
(*) CoronaMtl
(*) Diffuse = CoronaUvw + CoronaUvw + Bitmap
(*) Diasble High quality blending for both maps
4M Rays/s
This is the problem I found in my tests.