This, This and so much this. I love when I see people try to hype up this like its the 2nd coming of SSD tech. I guess all Sony's other proprietary tech was so much better than their counterpart UMD, Sony memory sticks, etc. This is just a way Sony can charge you god only knows how much for an SSD drive so you can't just go buy a cheaper SSD drive for your PS5. Well at least until someone makes an adapter like with the Vita memory cards.
The thing is, SSDs aren't all the same spec.
A low cost SSD is much much much slower than a 2019 high-end model and by all we heard up until now Sony intends to use something that uses PCIe 4.0 spec and wants the devs to be able to make full use of this speed. Insomniac made a 1 hour presentation on how they had to limit their streaming elements a ton because ppl were slapping low performing (yes, even lower than stock) HDDs into their PS4s - a scenario Sony seems keen to avoid this time especially as the rift between low end SSDs (hardly better than regular HDDs) and high-end ones is gigantic.