from what i know, smart phone is using camera "app" and running the app using its processor, so the processor has to run the app and save let say 8MP picture from your smart phone to your memory card and this take a lot of processing power
so when the smart phone can achieve zero shutter lag, it mean it has fast processor and good amount of ram, and good writing speed...another strategy to promote sales
but in the case of digital camera, it majorly depends on the memory card speed and the faster the card the lesser you fell the shutter lag, try a Class 4 card and a UHS Class 10 card, you know the kick, btw who want to use a camera with shutter lag?