We've now resolved the incident. Thanks for your patience.
We're currently investigating some performance issues with the Delivery Rates by Distance app. Our development team are actively engaging in the issue and will post updates regularly as the understanding of the cause and severity becomes clear.
To ensure checkouts are not impacted directly we recommended adding a static price rate to Shopify's shipping settings as a temporary measure until the issue is resolved. More information on how to do this can be found here: