I understand more sha-rounds make it difficult to bruteforce the hashed password in /etc/shadow. Higher rounds also use more CPU processing when executing
sudo commands and logging into the Unix account. But what are sha-rounds really? The
chpasswd manual doesn't really give a technical definition of sha-rounds.
-s, --sha-rounds ROUNDS
Use the specified number of rounds to encrypt the passwords.
The value 0 means that the system will choose the default number of rounds for the crypt method (5000).
A minimal value of 1000 and a maximal value of 999,999,999 will be enforced.
You can only use this option with the SHA256 or SHA512 crypt method.
By default, the number of rounds is defined by the SHA_CRYPT_MIN_ROUNDS and SHA_CRYPT_MAX_ROUNDS variables in /etc/login.defs.
The SHA-2 wiki doesn't really say either:
SHA-256 and SHA-512 ... use different shift amounts and additive constants, but their structures are otherwise virtually identical, differing only in the number of rounds.
In both the chpasswd manual and SHA-2 wiki, there isn't enough context to determine what "sha-rounds" are or how they relate to SHA512.