I'm reminded of LUKS Argon2 implementation, which is much slower than userspace. Therefore a sufficiently difficult challenge that may take less than a second in userspace Linux can take 20 or more seconds at boot.
You can see the implementation here: https://gitlab.com/cryptsetup/cryptsetup/-/blob/main/lib/cry...
In any case, it is GRUB which behaves slowly during boot. While I haven't tested Argon2, I believe it is affected in the same way as PBKDF2.