Am having the exact same issue. Have been using Aperture for years, but never experieced this bit of strangeness till a few days ago.
Selecting "Generate Thumbnails" under the Photos Menu seems to be a one-click alternative to viewing -- and then waiting -- for each image one at a time, but can anyone suggest why this very odd and always disconcerting behavior is happening at all?
does your camera create and embed high-resolution previews? Aperture will initially use embedded camera previews on import, if you have set Aperture use them in the Aperture preferences, but only if they have a high resolution, as explained in this document: Aperture does not use embedded previews from some cameras
(right now the link is not working - Apple's support site seems to be down)
How are your Preview settings? Perhaps the embedded previews that your camera generates are too small?