Henrik, sure the HDD is the slowest part in a system but once you have read a relatively small raw file you are done, the rest of the processing is carried out in cpu, ram, scratch and when working and saving in PS the disk write is much faster than the compression of PSD so while working I prefer uncompressed TIFF even when working to a HDD.
My system is a i7, 16 Gb, 3 separate SSDs for system, scratch, working files and HDDs for storage. 95% of the performance boost comes from that first SSD the others are added because I can in a midi tower.