That’s pretty much it right there 100%. I have no idea why AVX is likely the culprit, but it definitely explains why FX and later cpus have no issue. Up until then, SSE4a was the latest AMD had and sadly Phenom II was one year shy of getting it. Weirdly enough Intel and AMD both shipped CPUs supporting it at the same time. Intel shipped the Core platform with the i3, i5, and i7 first in Q1 2011. AMD followed with the FX in Q3 which sparked a lot of “add moar cores” memes.
nerd talk aside, I have no proof that it’s the culprit, but it seems very likely since it ends right when AVX was introduced to the markets. Someone with an Intel older than 2011’s Sandy Bridge needs to chime in, lol.