We guess the low attenuation is due to the stable tectonics, which also leads to relatively flat layering in the rocks. I think really the attenuation is due to the very low deformation rates, not the flatness of the structures. We think deforming rocks tend to accumulate damage, which softens and increases attenuation, while stable rocks slowly heal. However, there may also be an element of more scattering of waves by laterally heterogeneous structure than flat-lying structure.

Then there is both scattering and intrinsic attenuation, but that is getting too technical.

I don't see how increased wavespeeds would be caused by flat layers in any significant sense.