Add a .5s delay every time the checkout PMPro checkout button is clicked.

enqueue_scripts_slowdown_checkout.php

/*
	Add a .5s delay every time the checkout PMPro checkout button is clicked.
  This is meant to prevent automated scripts from testing your checkout page repeatedly.
  We will likely update this script to keep track on the server side as well to account for cases
  where the script reloads the entire page with every attempt.
*/
function enqueue_scripts_slowdown_checkout() {
?>
<script>
	function my_disablePMProCheckoutButtons() {
		jQuery('.pmpro_btn-submit-checkout,.pmpro_btn-submit').attr('disabled', 'disabled');
		jQuery('#pmpro_processing_message').html('Please wait...');
		jQuery('#pmpro_processing_message').css('visibility', 'visible');
		//jQuery('#pmpro_processing_message').show();		
	}

	function my_enablePMProCheckoutButtons() {
		jQuery('.pmpro_btn-submit-checkout,.pmpro_btn-submit').removeAttr("disabled");
		//jQuery('#pmpro_processing_message').hide();
		jQuery('#pmpro_processing_message').css('visibility', 'hidden');
		jQuery('#pmpro_processing_message').html('Processing...');		
	}
	
	var nsubmits = 1;
	var disablecheckout = false;
	
	jQuery(document).ready(function() {		
		jQuery(".pmpro_form").submit(function(event) {
			//every time the checkout button is clicked add a 1s delay			
			nsubmits++;
			
			//disable checkout now
			disablecheckout = true;
			
			//reenable checkout after nsubmits seconds
			setTimeout(function(){ console.log(nsubmits); disablecheckout = false; }, nsubmits*500);
		});
		
		//make sure submit stays disabled if we want it to
		setInterval(function() {
			console.log(disablecheckout);
			
			if(disablecheckout) 
				my_disablePMProCheckoutButtons(); 
			else 
				my_enablePMProCheckoutButtons();
		}, 500);
	});
</script>
<?php
}
add_action('wp_footer', 'enqueue_scripts_slowdown_checkout', 20);