There are various SSD types which have a huge impact on performance. SATA versus NVME used to be the big one, the interface, but now you have some NVME SSDs that are the same performance as SATA like the Intel 660 series.
Why is it slow? Well, it uses QLC NAND which is much slower than TLC, but configures some of it as a SLC cache, so the drive performs much better when it isn’t getting full. Cheaper SSDs often have no DRAM at all, or cut the amounts short, allocating less than the standard 1GB per TB of storage. The 660 series does the latter.
So just saying the PS4 and Xbox X will have NVME SSDs doesn’t actually tell us a whole heck of a lot about how the SSDs will perform.
But of course none of that really matters, because even the el-cheapo 660 SSDs are unbelievably fast compared to spinning magnetic disks.