defunct on "[Plugin: Markup (JSON-LD) structured in schema.org] High Server Load after upgrading from 2.3.2 to 2.3.3 or 2.4"

Sorry, I was mistaken, attachment_url_to_postid() is definitely an uncached function. I'll put in a pull request in the next few days.

In the meantime you should be able to just disable that code for now until an update is available. Edit this file on your install: https://github.com/miiitaka/wp-structuring-markup/blob/develop/includes/wp-structuring-display.php

on line 145:

if( $image = wp_get_attachment_image_src( attachment_url_to_postid( $url ), 'full' ) ) {
	 		return array( $image[1], $image[2] );
	 	}

comment the code out so it looks like this:

/*if( $image = wp_get_attachment_image_src( attachment_url_to_postid( $url ), 'full' ) ) {
	 		return array( $image[1], $image[2] );
	 	}*/

and the other functions should take over.