What watch are you looking at?

The fenix line, for instance, can be set to 1-second recording. GPS for speed is pretty responsive and almost automatic (when you stop at a red light for instance) and should be fine as long as you cycle in areas with good GPS signal.

A speed sensor would be better if you cycle where GPS signal is spotty (tall buildings, heavily wooded areas, tunnels ect). You will also need a speed sensor if you want to track and record the data on an indoor trainer.

Do you need a speed sensor? Depends on where you ride and if you want indoor trainer data. Do you need a cadence sensor? Again that depends on what you want. I do not use one as I don't care about cadence.