maybe PS5 'pre-caches' the last game you played on boot, so it 'loads' in 2 seconds?
just thinking out loud here
PS5 doesn't need to cache anything or to dump the RAM of the game into SSD space as Series X does.
PS5 will read from the SSD at ~8-9 GB/s and up to 22 GB/s depending on the case. Out of 16GB RAM, around 13.5 or 14GB should be available for games, so a PS5 game will fill the RAM in a cold loading in less than 2 seconds. And depending on the case, in less than 1 second (as an example, in an in-game loading where doens't have to fill all that RAM because part of it has menu stuff, basic stuff for the gameplay mechanics and main character, etc).
To this time you should add maybe some extra time to do some verifying with servers or maybe to create something procedurally in games like No Man's Sky or in certain huge open world games like the Ubisoft ones.
So if loading times for PS5 are like around a second or two long, I assume it doesn't make sense to waste ~100GB of SSD to store game cache dumping the RAM of up to 12 games on SSD as Series X to reduce that. Makes less sense considering PS5 will allow you to load the game directly in cold from the OS to a specific game mode, or in a specific custome challenge (like 'beat that lap time in certain circuit with a specific car') skipping menu and logo screens, in addition to load your savedata.