The man page of cryptsetup says:
Whenever a passphrase is added to a LUKS header (luksAddKey, luksFormat), the user may specify how much the time the passphrase processing should consume. The time is used to determine the iteration count for PBKDF2 and higher times will offer better protection for low-entropy passphrases, but open will take longer to complete. For passphrases that have entropy higher than the used key length, higher iteration times will not increase security.
Does it mean that a binary keyfile of more than 64 random bytes (each having 8 bits of entropy?) is useless with PBKDF2 and a hash of 512 bits like SHA-512? The cost of testing all the hashes directly being cheaper than testing all the keyfiles.
Does it also mean that reducing the iteration time to 1 millisecond (or just to 1 iteration if it's even possible?) for such a keyfile doesn't affect the security?
And finally does it mean that using PBKDF2 with such a big keyfile, by reducing its size to just 512 bits, actually lower the security of a LUKS header?