By default, Shopify apps load after the page is loaded. Since the page may contain heavy images or other integrations (e.g. Facebook & Twitter) it appears as if BookThatApp is taking a long time to load when in fact it hasn't even started to load.
To get around this, we've added the ability to load BookThatApp earlier so that it decreases the wait time.
1. From your Shopify admin, click Online Store to arrive at the Themes page.
2. Find the theme you want to edit, click the Actions ▼ button, then click Edit HTML/CSS.
3. Click on the theme.liquid file that's located in Layouts folder.
4. Paste the following code snippet above the </body> tag.
5. Save your changes.
There is a great article here on how to narrow down what is taking the most time to load on your site: How to Quickly Drop Seconds from a Site’s Load Time