The Develop module doesn't have "previews", per se. Pretty much everything that is displayed on screen is rendered on the fly each time by the CPU, even when just scrolling from image to image.
The Adobe Camera Raw Cache will save some of the base level rendering information (demoisaicing, etc) but it's really just a small part of the image that's being displayed and the rest needs to be rendered. Putting the ACR cache on a faster drive like an SSD will improve things, but by only a very small amount.
Really, the only way to significantly speed up the Develop module is to get a faster CPU.