As mentioned elsewhere, these chips only have a 128-bit SIMD unit (either for power or for space savings?) and so 256-bit SIMD executes half as fast as it would on Intel.
That seems to be untypically good, in the market where the new generations brought typically 10-20%.
I still wouldn't say it's impossible, as, for example, Intel had (apparently until Sandy Bridge) much slower NaN handling than AMD, at least without SSE2. So maybe AMD discovered some weak point like that. But until I read some explanation, it does seem to be too good...
