How about some SSD speculation? We had a rumor about the XSX using a Phison controller. If we assume it's a PCIe 4.0 controller, and I think it's safe to assume that, then we only have three options. The mid-range option is the E16, and we already have some SSDs on the market which uses it. One example is the Corsair MP600.
Here is the Corsair MP600 VS the launch Xbox One HDD - the Samsung Spinpoint Momentus ST500LM012:
As you can see, x44 times the sequential read speed (which matches Phil's 40 times faster statement) which is a good example for resuming a hybernating game out of a non-compressed memory dump (around 4 seconds to resume if we assume 13GB-17GB memory available for games) or initial loading of a game if the developer duplicates data and/or takes the time to arrange the initial loading data on the drive in a sequential order.
As for streaming randomly accessed data, like in Star Citizen (which
Dictator has pointed out), we get over 100 times faster data read which is an indicator of performance during the streaming of a large open world.