In my experience LR4 is slow by design. It did improve a lot having very fast HDD for the raw storage and running LR, its catalog and having all previews in a fast SSD.
However, LR5 runs faster to the point I find its speed sufficient. Loading the develop takes 1 or 2sec max and almost no delay while browsing the catalog (30k pics)
So my advice is get the fastest SSD you can for LR and catalog, the fastest dedicated HDD for the photos and upgrade to LR5.
And finally, maybe it is just me but I find the CR2 raws load a bit faster than DNGs. I used to convert to DNG at import, not anymore.