You have several kinds of drive activity going on here, and then non drive activity. Depending on your settings it may compress all the layers before saving first off. Your boot drive runs fastest when it's not full of stuff. As it fills up those write speeds dip. It happens on HDDs and SSDs although it's not exactly the same. Anyway with scratch space which is a dedicated photoshop pagefile (you can check the size in a number of ways) if it's having to check that while writing the file, it's constantly having to look at different parts of the disk. This can slow things down significantly. On either a raid volume or SSD I doubt you'll be held up if you just have scratch set to your boot drive assuming adequate ram. If you're lower on ram and everything is assigned to the boot drive, it slows things down immensely.
The point to all of this is that if you have zero bottlenecks and a fast computer, these things can save really damn fast. If you're using a larger hard drive for your saves, I can compile a list of suggestions to speed things up later starting with turning off layer compression. That will save you at least half the time you mentioned right there.
