Since scripts registered by Shopify apps' are loaded once the page is loaded successfully, there is only one way to speed it up. Add following line of code in your theme.liquid before </head> tag:
<script src="https://cdn.shopify.com/s/files/1/2325/8929/t/3/assets/ultimate-countdown-timer___app.js?shop={{shop.permanent_domain}}&t={{ 'now' | date: "%Y%m%d"}}-1" type="text/javascript" async="async"></script>