So I know many people use this trick already, but I've never noticed much of a difference when I've done it before.
Tonight I was in the middle of experimentally moving my images to a fast drive, and had a quick play with the images disconnected from the previews in LR.
(In other words LR was using the smart previews only). The speed difference is huge (its still not great but its a least usable). I suggest anyone who has installed 6.6 simply import photos, generate smart previews, then rename the folder that contains the raw images.
Also now appreciate that the manual transforms are available in the upright transforms without a mouse click.
To get quick speedboost all adobe would need is a big fat red "make me go faster button" that forces LR to use smart previews.