This RC is definitely a step forward with respect to the responsiveness of the develop module. Whilst it's still not snappy and there is a lag between the screen updating after the sliders are moved, it is finally workable.
It also went through my catalogue and retrieved tone curve settings, curious as I had already run the recovery script on 4.0, but this time it worked better as previously it had mis-reported some files as having point adjusted tone curves that had no modifications.
Contrary to the comment above; this isn't a beta, so doesn't install beside a current 4.0 installation. It's a release candidate, so it replaces and overwrites LR4.0. I've not found any problems with it yet here, but it may be wise to take the usual precautionary measures of ensuring a good back up before installation.
Rather a shame they didn't have this in place at release, it was well reported when in beta.