Reject Comments Submitted Too Fast

// Minimum seconds between loading the page and submitting a comment.
$wpide_comment_min_seconds = 5;

add_action( 'comment_form', function () {
    if ( is_user_logged_in() ) {
        return;
    }

    $time = time();
    printf(
        '<input type="hidden" name="wpide_ct" value="%1$d"><input type="hidden" name="wpide_cs" value="%2$s">',
        (int) $time,
        esc_attr( hash_hmac( 'sha256', (string) $time, wp_salt( 'nonce' ) ) )
    );
} );

add_filter( 'preprocess_comment', function ( $commentdata ) use ( $wpide_comment_min_seconds ) {
    $type = isset( $commentdata['comment_type'] ) ? $commentdata['comment_type'] : '';

    if ( is_user_logged_in() || ( '' !== $type && 'comment' !== $type ) ) {
        return $commentdata;
    }

    $time = isset( $_POST['wpide_ct'] ) ? (int) $_POST['wpide_ct'] : 0;
    $sig  = isset( $_POST['wpide_cs'] ) ? sanitize_text_field( wp_unslash( $_POST['wpide_cs'] ) ) : '';

    if ( ! $time || ! hash_equals( hash_hmac( 'sha256', (string) $time, wp_salt( 'nonce' ) ), $sig ) || time() - $time < $wpide_comment_min_seconds ) {
        wp_die( 'Your comment could not be submitted. Please go back, wait a moment and try again.', 'Comment rejected', array( 'response' => 403, 'back_link' => true ) );
    }

    return $commentdata;
}, 1 );