I'm in the process of doing pre-Beta testing, and I'm running up against this as well.
Saving a game state to a save slot takes 12-15 seconds on actual hardware (I do prompt the user that 'this will take up to 15 seconds, please wait' but on hardware I get the infamous crash.
I suppose I could (try?) to break up the save into chunks (it's already multiple JSONs, so that's possible) and kludge with juggling semaphores etc. but it seems like a lot of trouble to go through when I know that the game 'isn't hung' but is just going to take >10 seconds.
As games get larger on Playdate (hint: once we get out of Beta 
) IMO there should be some way to (temporarily?) flag that a long I/O is about to happen and that the Playdate runtime shouldn't freak out.
Hmm. Will still post this here, but will also put in a Feature Request for same.