Extract from Adobe Lightroom 2 – The Missing FAQ
There are a number of options to choose from when importing your photos, and which you choose will depend on your own browsing habits.
Minimal shows the thumbnail preview embedded in the file. It’s the quickest option initially, but it’s a very small low quality preview (i.e. usually with a black edging and about 160 px along the long edge) so you then have to wait to render previews as you browse. Minimal previews are not color managed.
Embedded & Sidecar checks the files and their sidecar files for larger previews (approx. 1024 px or larger), giving you the largest ready-built preview it can. It’s still just a temporary option – Lightroom will build its own previews as soon as it can.
Standard builds a standard-sized preview used for browsing through the photos. You set the size and quality of these previews in Catalog Settings. Standard-sized previews are highly recommended – it will greatly speed up browsing performance if Lightroom isn’t having to render previews on the fly.
1:1 previews are full resolution so they take up more space, but if you wish to zoom in on your photos in Library module, it will save Lightroom having to render 1:1 previews on the fly, which would slow your browsing experience. If you’re concerned about the disc space that they take up, you can set them to delete after a fixed time (in Catalog Settings), or you can discard 1:1 previews on demand by selecting the photos and choosing Library menu > Previews > Discard 1:1 Previews.
You can either choose to render either Standard-Sized or 1:1 previews in the Import dialog or if you wish to render the previews later, select all (or none) of the photos in Grid view and choose Library menu > Previews > Render Standard-Sized Previews or 1:1 Previews.
In early version 1 releases, rendering previews at the same time as importing was much quicker than rendering them later, but this difference is no longer noticeable.