The velocity in the port or valve is a function of the open area and the dwell (open-time). Getting air into and combustion gases out of an engine requires a pressure drop. The required pressure drop increases as the engine speed increases. Larger valves reduce those pressure drops and parasitic losses. Likewise, the longer a valve or port is open, the lower the pressure drop. Each cylinder has three chambers. Only the combustion chamber is limited by time.
Increase the valve or port open-time will decrease the time for the power stroke. This becomes a design tradeoff.
The axial flow combustion chamber has ports at one end and a large poppet valve at the other end.