Strangely I've been having the opposite experience, performance for me has been getting better and better and I have now got almost every setting on Ultra or High (I am only at 1920x1080 though), its running smoothly for me now and doesnt look too bad at all as you can see from my screenshots in the thread. (GPU is a 1070, cpu is an i5-2500)
Tbf, I can't rule out that it IS my imagination
Or just that I'm just noticing the lower frames more, perhaps since I played Sunset Overdrive (with its fast-pased action with high FPS) since I first started RDR2.
But I did find it odd that I dropped down reflections, SSAO, far shadows, volumetrics and a few others the other day and still only get low to mid 40s. I feel I would have noticed even lower frame-rates in the past when I had those settings higher.
It looks like I'm going to have to drop quite a few more notches on things to get up to 60ish fps.
It still looks really good, but I am starting to wonder how low I'll have to go with things now.