Latency is not zero. What's more latency will never be zero unless we invent completely new communication technologies. We currently have no communication method that operates at faster than light speed. At light speed it takes 30ms to get from one side of the Atlantic to the other (not the furthest your web data may have to travel). Those 30ms slices add up, and that's assuming you've got a direct and perfect connection to your users. You don't, so chances are the best latency you can cope for is much longer than 30ms.