The ‘::PostOpaque’ performance (details in post) is solved by changing the GTX 670 (2Gb) to GTX 1080 (8Gb) Gaming X. I was told by the Simul guys that the happens in older graphic cards. There’s something they just don’t support but the actual reason what is causing the is indefined and propably will never be determined, follows from their words.
The is not happening on 1080’s. 1060’s and also they tested it on GTX 980. So, there is some step between 670 (and below) and 980 (and above) where the starts happening.