Woocommerce Checkout Creating Multiple Orders and Running Very Slow

Hi All,
Hoping the forum can help with an issue I’m having. Our checkout page (https://americanmadetactical.com/checkout/) is running incredibly slow. In addition to it running slowly, when a customer places an order the page will just spin.

Eventually, the page will reload with all fields blank. The order gets submitted to us, but the user doesn’t think it did and they’ll enter their information again. This is resulting in unintentional multiple orders.

We’re not sure if this is a theme, stamps.com plugin, or woocommerce issue. Could it be because our PHP version is 5.4.42 and not 5.6 as recommended? Any help would be greatly appreciated.