Operating system: Windows 10, Ubuntu Linux 18.04
Graphics card: Geforce GTX 1060, Geforce RTX 2060
Broken: every Blender version except 2.79b
In simpler scenes Cycles performs much worse as it should. It affects the render preview, progressive refine and tiled rendering in final render also. There is already a long thread - with many misconception - made by myself here: https://blenderartists.org/t/cycles-much-worse-performance-with-rtx-cards/1149075/65
Finally one of the Blender Developer named Team Stefan Werner pointed out (reply 65) what might be the real problem:
"I gave that scene a quick spin in the profiling tools, and it does indicate that my RTX card is spending most of its time idling, the CUDA cores are waiting for instructions to be loaded from memory. In other words, that scene is too simple for Cycles’ code and the GPU is slow at skipping over unused code.
Nvidia writes that their GPUs have instruction caches but does not disclose any details - it may be that those changed between the10x0 and the 20x0 cards and that the 10x0 series was just a lot better at dealing with Cycles’ megakernel."
He also mentioned a workaround (Adaptive Compile), but it is broken too, therefore unusable. It is Linux dependant and not working correctly.