I use Silkypix Developer Studio Pro and before that Silkpix 3.
From scratch when opening a new folder of raw files it takes no more than a second to generate a preview, any adjustments happen in real time with hardly no delay except for altering sharpening pre-sets at 100% veiwing, this takes about 3 to 5 seconds on a 12Mb raw file.
I own LR2.6, and Capture One 5, and Silky pix is still my favourite raw converter.