It looks that Pentax 6×7 shutter shake is not that serious problem, anyway – it depends on lens and other factors. Below is sample of a photo, produced handheld. In my opinion there are no traces of shutter shake, at least for lens 90mm.
Photo: pentax 6×7, handheld, without MLU lens: 90mm LS Takumar Super-Coated lens shutter speed: lens 1/250s, f8 focal shutter speed: 1/4s film: fuji acros neopan 100
Magnification of the circled area from the left photograph – arrows point to visible slim metal aerial and aerials elements. No trace of mirror and/or focal shutter shake. It seems that the Pentax 6×7 90mm LS lens is not so bad, anyway if resolution is considered.