>I haven't found any asynchronous IOPS numbers on HDDS anywhere.

As the other user stated, just look up Crystal Disk Info results for both HDDs and SSD and you'll see hard drives do about 1/3rd of a MBPs on random file IO while the same hard drive will do 400MBps on a contiguous read. For things like this reading a zip and decompressing in memory is "typically" (again, you have to test this) orders of magnitude faster.