I think the assumption is that this is going into a somewhat modern hashing algorithm like argon, bcrypt (created 1999 - that's a quarter-century ago), or scryp

I think the assumption is that this is going into a somewhat modern hashing algorithm like argon, bcrypt (created 1999 - that's a quarter-century ago), or scrypt with salt. With those assumptions, the calculations aren't reusable, and definitely not 1B passwords / second. If that's not true and the password is being stored using MD5 (something that's been NIST-banned at this point for over a decade), then honestly all bets are off, and even 128 bits of entropy might not be enough.