AVX gains can be very tiny even with x265, but this might depend on the video and settings. Not long ago I've compared AVX2 and non AVX x265 binaries and the difference on my i7-7700k was not more than 2-3%. Imho x265 is not a showcase for AVX performance.
I downloaded today's build from
builds.x265.eu and ran some tests on a Haswell processor. All encodes are at 1080p.
x265-10bit --preset veryslow:
x265 [info]: HEVC encoder version 2.4+26-355cf3582263
x265 [info]: build info [Windows][GCC 5.3.1][64 bit] 10bit
x265 [info]: using cpu capabilities: MMX2 SSE2Fast SSSE3 SSE4.2 AVX AVX2 FMA3 LZCNT BMI2
encoded 300 frames in 291.83s (
1.03 fps), 1924.72 kb/s, Avg QP:30.30
x265-10bit --preset veryslow --asm SSE4.2,FMA3,FMA4,BMI:
x265 [info]: HEVC encoder version 2.4+26-355cf3582263
x265 [info]: build info [Windows][GCC 5.3.1][64 bit] 10bit
x265 [info]: using cpu capabilities: MMX2 SSE2Fast SSSE3 SSE4.2 AVX FMA4 FMA3 BMI2
encoded 300 frames in 406.18s (
0.74 fps), 1924.72 kb/s, Avg QP:30.30
AVX, i.e. 256-bit vector unit, increased the performance by 40%, which is more than reviewers showed (20%), since they only ever use the fast presets that do very little analysis. These results are consistent with findings on the Doom9 forum. Perhaps the "AVX2 and non AVX" binaries you downloaded were referring to compiler optimizations, which would not be expected to affect the performance of assembly code.