Order Confirmation emails taking hours to send

We are working with one of our app’s customers - they are having an issue where their order confirmation emails are not sending for 2+ hours and it is causing their customers to think the order didn’t go through.

The customers then submit duplicate orders.

These orders are $75,000+ on a Shopify Plus store.

What controls the timing on these emails, and is it something that can be fixed or would we need to involve Shopify Support?