Function call time time may increase after function has been idle for some time or is being called for the first time. It happens due to the fact that we need to load the functions before executing it. Time after which functons are unloaded is not fixed and is subject to constant adjustment. Think tens of seconds to minutes.
You are charged only for the call time and not for of infrastructure latency. Note that request times on the client may differ from the ones observed in billing due to network latency and throughput.