Is anyone seeing this behaviour?
Specifically; click on Photos at the top of the Library Inspector, to show all the photos in your library, then go into Viewer/Thumbnail Mode (so you just see all your thumbnails in the Aperture Viewer window). Open the Activity Window. Now scroll fast up or down through your photos, but occasionally allowing the scrolling to come to rest for a couple of seconds.
Are you seeing in the Activity Window, that previews are being generated?
I am seeing this with RAW files from my D800e. I’d be interested to know if anyone else here who has this camera is seeing the same behaviour. (Or of course, if anyone sees it with any other camera). I’m uncertain if it started with the 3.5.1 update, or another update that I recently installed at the same time.