How does carrier API response time impact rates returned to my Shopify store?

Shopify allows a 10 second window for a response back to the store. The carrier response may not take more than 5-7 seconds.

Ten seconds is the total window of time that Shopify allows for 3rd party negotiated rates to be returned and calculated according to Merchant settings.  There are many operations that have to be performed in addition to the negotiated rate.  For example: the address type has to be confirmed, Standard Box Sizes (SBS) must run when enabled, the Google Cloud Platform has to be consulted to get the distances so the right warehouse is selected, etc.  All of these are separate API transactions in addition to the carrier's response.