On my case the user has to specifically save is profile to disk and then extract/save the nsec to elsewhere. Most users will never do this, they just visit with a instantly created account to play around and never use it again.

I'm eagerly waiting for the nostr:nprofile1qqs2prxxu55dssm0eqkpadnuw0q5tx0dv0w4xnr0smh30gnsr90sdtspr9mhxue69uhhyetvv9ujumt0d4hhxarj9ecxjmnt9uexa2ly team to announce battery/processor optimization. It's going to be a game changer.