VeraCrypt's PIM is unnecessary if you use a sufficiently strong password.
What VeraCrypt's PIM is
In layman's terms, VeraCrypt's PIM defines the number of times your password is hashed before being used to decrypt the disk.
To be precise, each VeraCrypt volume is encrypted using a random master key. Your password is used as a base to decrypt the master key: what actually decrypts the master key is the result of a key derivation function which takes your password as input. This key derivation function repeats its internal hashing computation a number of times depending on the PIM.
From the VeraCrypt manual:
When a PIM value is specified, the number of iterations is calculated
- For system encryption that doesn't use SHA-512 or Whirlpool: Iterations = PIM x 2048
- For system encryption that uses SHA-512 or Whirlpool, non-system encryption and file containers: Iterations = 15000 + (PIM x 1000)
Implications of VeraCrypt's PIM
VeraCrypt's PIM increases both the time it takes you to decrypt the disk and the time it takes an attacker to brute-force your password. The stronger your password is, the longer a successful brute-force attack would take, the lower the PIM can be to make the attack impractical or anti-economical. Therefore if you use a sufficiently strong password then VeraCrypt's PIM is superfluous.
Using VeraCrypt's default PIM increases attack time and decryption time proportionately, while using a custom PIM increases attack time more than it increases decryption time, because the attacker would have to brute-force both the password and the PIM. The time increase of the attack in the last case is 1+2+3+...+PIM = PIM(PIM+1)/2 corresponding about to a square.
Assuming your password only uses English letters and numbers, increasing the PIM by a factor of 1000 (e.g. 500 --> 500000) roughly corresponds to increasing the password by just 4 characters (36^4 ~ (1000*1001)/2). In my opinion it is counter-productive because you need to memorize about the same number of characters with the added nuisance of increased decryption time.
All the above is unrelated with AES, which is a symmetric cypher used to decrypt the VeraCrypt volume after a random master key is derived.