Nor sure about that. My first complain was about browsing RAWS, 1/1 previews, so it's probably not about disk read/write speed, but more about processor and RAM. This has been discussed before, and from other posters experience, there is not much to do about it, except using another software for the first step, preview and selection of large quantities of RAWS.
what CPU you got? SSD would improve any build by huge margin, the slowest part of lightroom is database/catalog.