Quote from: Sanglotslongs2 on Today at 06:21:56 PM

This "passphrase" is (I guess) "custom word" in electrum and "BIP39 Passphrase (optional)" in https://iancoleman.io/bip39/ ?

Yes, they are all the same.

Quote from: Sanglotslongs2 on Today at 06:21:56 PM

This is an equivalent for PBKDF2 iterations ?

No. Encryption is not the same as just increasing the number of iterations.

The real question is "What are you trying to achieve?" If you want a set up which means your coins will not be immediately stolen if your seed phrase is compromised, then you should use a passphrase, since this is a BIP standard and reproducible across multiple wallets. Changing the iteration number is both less secure than a complex passphrase and non-standard and could cause you problems in the future when trying to recover your wallets.