https://www.tweaktown.com/news/7134...ep-dive-into-next-gen-storage-tech/index.html < Give this a read, it will give you some general information on how the PS5's custom flash controller can manage to push assets quickly from storage->RAM->CPU.
To tl;dr it though, basically the custom flash controller used in the PS5 can push around 5gb/s-9gb/s, meaning 5-9GBs worth of assets can be loaded to the RAM in each second, and that data can then be loaded by the CPU/GPU basically instantly (since the RAM's bandwidth is around 450gb/s, and they're using a custom chip that can decompress data quickly if the data has been compressed). As Cerny mentions, you can essentially load 4GB worth of data from storage->RAM->CPU in a single second, that's basically a good "average case scenario". The storage speeds aren't quite fast enough to load data directly to the CPU, but it is fast enough to load lots of data to the RAM which can then push lots of data to the CPU in basically an instant when it needs to.
Because games (at this time, anyways) don't use a huge amount of assets, and because the system can load so much data in just one second, load times can be virtually eliminated...if the developer uses the hardware properly, of course (which a few launch titles didn't, AssCreed Valhalla being one of them which has equivalent load times to PCs). And, for games that utilize the hardware, this is basically true. Demon's Souls Remake, for example, can go from the main menu to the game world in something like 5-6 seconds, and then from game world to game world it's basically 1-3 seconds.
The question about an off the shelf NVMe being fast enough is a valid concern, because if that NVMe can't push at least 4GB/s for a decent amount of time, it could very well have performance issues with games that are designed to take full advantage of the PS5's internal storage speeds. I suspect this is why Sony doesn't have that M.2 slot enabled at launch, because there aren't enough PCIe 4 NVMes currently available to buy that can support those kinds of speeds (and the ones that do exist are expensive).
However, I 100% expect that you can't just shove any off the shelf NVMe in there and call it good, Sony will either create a hard whitelist that won't let you use a slower drive at all, or they'll implement some kind of benchmark that must be passed before a drive can be used anyways to avoid those performance issues and so it's not something the end user will have to worry about so long as they purchase a proper drive.