What do people mean when they say that a lens' performance degrades poorly at larger apertures like f45 to f64 on 4x5, or f90 to f128 on 8x10? After reaching their optimal point, all lenses will lose performance when stopped down due to diffraction.

So can it really be the case that some lenses are excellent performers at f/16 - f/22 (vs. other lenses as a % of the diffraction limit), but poor at higher f-stops? That would be using the light only from the central part of the lens then.

Seems counter-intuitive unless the aperture blades are awful (I've heard diffraction explained as caused by the percentage of light-interaction with the aperture blades...but I've also heard an information-theory argument that even if "God himself" took away light beyond the central part of the lens, it would still lose resolution...less information in = less information out).