The maximum response time is currently 15 seconds. If you app takes longer than that to respond then a timeout will occur and the rates will not appear at checkout.
You are operating a service. You are told you have to respond within 10-15 seconds or you are cut-off from presenting your results.
How is that a problem? If you cannot figure out a shipping rate in 10 seconds and return it, your service is too slow and needs to be fixed up. Since all major carriers offer API services, and you never sit around for 15 seconds waiting for rates from them, it makes some sense to lump everyone into that range of performance.
Allowing merchants to control this time-out is ludicrous. Imagine your service is so slow it takes 15-25 seconds to return a shipping price? What is your timeout? 25 seconds? No one will wait 25 seconds for shipping rates. They will bail out on a broken site, as that is broken.
Well, my APP displays results from all Shipping Carriers and I have set a 10-second timeout to collect rates from different Carriers. So if I chose to wait for all Carriers (in scenarios where one or the other Carrier times out), the APP takes around 12-15 seconds to respond - But Shopify is timing out even this response.
However, everything was good until Saturday morning- there was not even a single time-out for my APP's rates.
I read somewhere that Shopify has reduced the Time-out from 15 seconds to 10 seconds.