As an example, I had slowdowns when viewing an 8MB PDF file. I used Save As to create a copy, which was 24MB in size. But when opening this much larger file, I experienced no such slowdowns.
[robg adds: The hint's author sent me his troublesome 8MB PDF file, and it definitely had speed issues in Preview, even on my Mac Pro. Thumbnails appeared at a glacial pace, and zooming in and out was painful. I used Save As, opened the now three-times-larger file, and the change was quite noticeable. Zooming, which used to take a couple of seconds, was now well under a second. When I scrolled by dragging the scroll thumb, Preview didn't lag nearly as badly as it did with the original. I theorize that the increased size and better performance are related to removing compression from images, but that's just a guess -- if you have other theories, please post them.]