There is some big confusion spread here....
There is nothing bad with the 5400 rpm speed as such. Recording density has increased recently sufficiently, that a 5400 rps drive is able to sustain 150 MBps read/write, while a 10-15,000 rpm drive is able to sustain say 300 MBps read/write.
However, this only applies to sequential operations, where the drive can prefetch data in the cache (data is read form the media at much higher speeds).
The big issue is the 'green' drives. These drives save energy, bu.. not using it
How a drive does not use energy? It can do this in many ways. Spinning the platters at lower speed is only one of these. Other ways are to use slower and less power hungry electronics. SAS drives have more complex electronic assemblies and the same drive in SAS version consumes more, that with SATA. Of course, the performance is worse. Another, very big power drain in the drive is the head assembly movement. It is simple: if you want small reposition times, you have to waste more energy. One significant way to save energy in the 'green' drives is to make the head assembly move slower, not as aggressive so they have abysmal random seek times. There is almost nothing else a drive does. Maybe, a 'green' drive could shut off parts (motors, chips) but this reflects to the ability to respond quickly to requests.
In a single-user system, or in systems with predictable sequential load any drive will perform well. In a multi-tasking system, when the drive is asked for data scattered all over, things are much different. I still have some old Cheetah drives here. Compared to a green drive, they are better for random type load, but cannot compete on sequential taks, with their 'poor' 30 MBps
So, in summary, while the improvements in recording density have diminished the difference between slower spinning and faster spinning drives in terms of sequential load, the higher end drives spend power on more critical for performance tasks, such as seeks and faster electronics. It seems recently, it is the electronics that is the limiting factor for drive performance. When we talk about 'slow' drives, we usually talk about the random access times and multitasking performance of the drive.