I would not worry about delay. It is acceptable in API based computing to experience delay. If you use other API based services you'll see that responses can take anywhere from 300ms to 1500ms to 5000ms. It is probably not you. Have you run a tracert between your server and Shopify?
The whole point of this is that it should not matter how long it takes. As long as you code defensively to respect the 500 API call limit in 300 seconds, you're golden. When Shopify servers are not under high load your calls will complete faster than when Shopify is being hammered, which can happen.