_doing_it_right.php

<?php

namespace JPB\Yassss;

add_filter( 'wp_insert_post_data', __NAMESPACE__ . '\\replace_values' );

function replace_values( $post_data ) {
	$content = empty( $post_data['post_content'] ) ? '' : $post_data['post_content'];
	
	$content = str_replace( '{{ my_marker }}', some_really_long_running_function(), $content );
	
	$post_data['post_content'] = $content;
	
	return $post_data;
}

_doing_it_wrong.php

<?php

namespace JPB\WTF;

add_action( 'save_post', __NAMESPACE__ . '\\replace_values' );

function replace_values( $post_id ) {
	$content = get_post( $post_id )->post_content;
	
	$content = str_replace( '{{ my_marker }}', some_really_long_running_function(), $content );
	
	remove_action( 'save_post', __FUNCTION__ );
	
	wp_update_post( [ 'ID' => $post_id, 'post_content' => $content ] );
	
	add_action( 'save_post', __FUNCTION__ )
}