From what I gather it's both. The lens' aren't designed for CDAF, so they're both noisy, and slow. However, it's also true that the additional pins of CDAF designed lens' mounts offer far better communication for the CDAF. I guess that pretty much lumps into the other category, as the lens' aren't designed for it. CDAF requires very fast communication to and from the lens to work at high speeds, since the lens needs to shimmy back and forth to pin point the exact focus point.That'd be the first I've heard of it. I was under the impression that the problem mostly stemmed from the lenses not being designed for it, and thus being slow and noisy.
Also it's not just the focus motors that companies such as Panasonic/Olympus/Sony/Nikon/etc.. are changing in thier lens' to get them to CDAF fast and silently, they're also designing the lens' designs/groups so that the focusing elements are as light and nimble as possible.
I was hoping that when the time came, we could just send in our PDAF lens' and have them retrofitted for CDAF performance. Unfortunatly, new lens' are required instead.