It’s not so much about time (although I have played a couple of things that would take a ridiculously long time to save or load), it’s about the number of chances to make a mistake. If you only save ten values, it’s really easy for a programmer to verify that they’ve got everything right, but if they save ten million, there are a million times more opportunities for mistakes to sneak in and it’s much harder to notice each mistake, let alone fix it.

Fallout 4 is a bit of an odd duck here as the save format for the BGS games is basically just another ESM file, so reuses all the same serialisation and deserialisation mechanisms. Most games don’t have multiple places the game data can come from and a way to combine them as they’ve not got an engine designed with this kind of modding in mind, so there’s nothing to reuse in this way for saves. Given the general standards of engineering from that studio, if they didn’t have this as a core feature of their custom engine for nearly three decades, and instead had to come up with something from scratch, they’d absolutely mess it up or have to simplify the saving system.