I experimented with the preview quality option set to High for a while. The previews were huge (I have 1680 pixels selected because I have a 1680 x 1050 panel, and I don't want Lightroom generating 1:1 previews unnecessarily) and Lightroom got quite sluggish.
In the end, I changed the setting back to Medium, quit Lightroom, deleted the preview folder, restarted Lightroom and rebuilt all my standard previews. The previews finished up taking less than a quarter of the disk space (1.4GBytes rather than 6.2GBytes - in both cases I'd discarded all the 1:1 previews). Lightroom feels a fair bit snappier as well - though maybe that is a placebo effect!
David