When encrypting large files (without compression) the difference between GnuPG and Kleopatra becomes significant because Kleopatra needs around 5-6x more time than GnuPG. That's why it would be great to increase the performance of Kleopatra.
Edit 2024-01-11 (ikloecker): The problem exists for all crypto operations (sign, encrypt, decrypt, verify). For OpenPGP the problem is tracked by this task. For S/MIME it's tracked by {T6928}.