There are some different options on how to build and/or display previews. Not sure how all this works but right now I have the default setting. In Library view, (Grid) when i scroll down the thumbnail looks very grainy and then slowly imrpoves its resolution. In iPhoto the thubnails don't change resolution on the fly and always loook good. So in that way, IPhoto seems better than Lightroom to me, in this area at least.

If I understand this right, thumbnails in Grid view are small JPGs of a larger RAW file, and if I bring the image into full screen then I'm looking at an unmodified RAW file with "filters" in front of it to show whatever edits have been applied such as exposure, white balance, etc.

Is there a way to get higher res JPGs or to store them for longer than 30 days? Do I understand correctly that doing so will make things run slower?

(I have a good Mac with lots of RAM etc.) So this is part question and part to confirm if I "get it yet".

Thanks

The thumbnails that you see are initially pulled from the JPEG embedded in the RAW file, but will be updated & replaced by the previews/thumbnails that LR builds by itself, directly from the RAW data, including your edits. It just takes some time to build these previews, and these previews will be stored on your hard drive, indefinitely. There is an option to have full-sized (1:1) previews tossed out automatically after some time (e.g., 30 days, or some other duration of your choice). This is an option since 1:1 previews occupy much more space.