I couldn't replicate that difference between the two versions on my 3700X. I used the VS 2019 AVX2 builds from
http://www.msystem.waw.pl/x265/
With 3.1+7, encoded 2000 frames in 461.37s (4.33 fps), 3467.58 kb/s, Avg QP:20.97
With 3.1+8, encoded 2000 frames in 459.33s (4.35 fps), 3467.58 kb/s, Avg QP:20.97