As of October 10, 2025, the Cart AJAX API and Storefront Cart GQL API now processes simultaneous requests. For example, when sending two cart add requests at the same time, the Cart AJAX API executes both requests.
This is a companion discussion topic for the original entry at https://shopify.dev/changelog/improved-concurrency-handling-in-the-cart-ajax-api-and-storefront-cart-graphql-api