Hi Stan,
I have a25b-4 version of the game from official Manjaro repositories.
I played with graphics settings and found out that it's very high shadow settings that kills fps after 4096px window width. Switching off GLSL or postprocessing didn't make game playable when shadows were very high. If I use just high shadows then I have over 100 fps with 5120x1440 but when switching to very high it decreases fps at least 20 times.
Just before the slowdown GPU memory usage and GPU utilization are around 30-40% and PCIe bandwidth around 20%. When I resize window over the limit then these numbers are 40%, 100%, 95%.