Unfortunately, this is only half true. Meanwhile I have run further tests because RTX 2080 should be faster than 5700 XT. RTX 2080‘s chip has about 1.3 times more transistors (billions: 13.6 vs. 10.3) on GPU die and Nvidias’ cards drivers are currently more optimized than AMD’s. Well, RX 2080 price is about twice the 5700 XT’s. VRAM of both cards has the same capacity of 8 GB.
This numbers are very odd. I have no explanation for speed decrease after 13 minutes rendering. Speed gets more constant after 36 minutes and prediction of render end time at 57 minutes is close to observed time: 2h06m. (I have observed render speed decrease with my own scenes too sometimes.)