I created a new catalog, selected the preview size of 1024, medium quality, then imported a bunch of Nikon NEF photos, and converted to DNG. Regardless of whether I render previews on import, or render previews manually, when I scroll through Loupe, I get the Loading message. When I look at the previews folder for the catalog, the preview for a sample photo changes from about 250KB to 1.5MB after I get the Loading message.
When I create another catalog and select a preview size of 1440, it works as expected and there is no delay scrolling though Loupe mode. The previews seem to be, however, about 400KB in size.
So there seems to be 2 issues. One is that the 1024 previews aren't generated until the photo is viewed, and the other is that the size of the preview is disproportionate.
Anyone else see this?
System is Windows XP SP3, Lightroom 3.2.
What's your screen size?
I think Lightroom will build the standard previews on the fly, if the pre-built previews are smaller than is required to fill your screen. That explains why you're seeing the Loading message when you have only 1024 previews rendered — Lightroom re-renders standard previews at a larger size (probably even 1:1).