My theory is that, without optimization, ps4 games won't load in 1 second on the ps5.
I think this is why Cerny said over a hundred ps4 games are tested to be BC. He probably meant they won't be BC only - they will be optimized for the ps5's standards.
They mentioned that aim to have most PS4 supported by BC, 4000+ out of the 4000+ available games, Cerny just mentioned that recently before his talk they tested the top 100 most played ones, which doesn't mean they were the only ones tested until now or that they were the only ones available at launch. These games need to be tested manually one by one, because like in any emulator or BC some of them may experience issues. so in this case Sony would suggest the devs of the games (I assume in the cases the studio is still running and has time and budget available for this).
They didn't provide enough details, but I assume these issues may range from frezzes and crashes to loading issues and visual glitches. That may appear in BC boost mode (if exist and available like in PS4 Pro, these would be sub 1s loadings) base BC PS4 speed mode (same loading speed than in PS4).
And month by month they will be adding more optimized ps4 games. Because they probably don't want a state of decay 2 situation. They don't want a ps4 game to run five times faster on the ps5. They want it to load instantly. Every single one of them, eventually.
But I think they will optimize every single one, except for the incredibly simple ones like Not a Hero or Downwell, which could insta-load on the ps4 on the get-go.
If the game is coded properly to have scalable loading times (the multi games are more likely to be ready for this), raw PS4 games would load in less than 1 second if there is some sort of boost mode. I assume it will be the case of most games. Then maybe a few hundred games may experience issues with this, or may need other types of fixes to work properly in the PS5 BC. But only the publisher/dev of the game can change them, and maybe this studio or publisher shut down the company in the past, or are too busy working with something else, or they decide that don't want to spend a budget in a game that no longer will generate revenue for them because almost nobody uses BC or buy previous gen games.
There are people who claim he actually said it runs at 40fps in the editor on his laptop.
The question is is that true? And if true at what settings? Some say 1440p others say 1080p.
In any case the fact that they had to do careful data optimization layout on the ssd for the flight area suggests that even the laptop would have struggled otherwise even with its likely state of the art nvme. Such optimizations are impossible on open world, so at least that laptop with its current software seems like it would struggle with open worlds at this level of detail at least until further optimizations are made.
1440p was the native resolution of the PS5 demo. To run it in a laptop with a way slower SSD at higher framerate it should be running at a way lower resolution or detail. I think it was both 1080p and less quality level.
In theory this engine makes the streaming that scalable: it sales down the detail of the scene to the hardware capabilities where it runs. As an example, the demo we saw in PS5 was 1440p 30fps, but they could have put it at 4K 60fps, but this would have reduced the detail quality of what we saw.
Same goes when running it on a worse hardare (SSD/GPU/SSD...), they may also run the demo but reducing its quality detail.
In any case, they are a year and a half of the final Unreal Engine 5 release, so they have still a lot of work to do including optimizations, fixes and completing stuff. It's very likely that once completed this demo will run way better on PS5, next gen PCs and Series X than we saw in the demo.