If a step in the temperature of the entering air occurs, there will be a lag time before this temperature change appears at the air outlet, can be sensed by the control system, can result in a change in valve position and thus a change in thermal input to the exchanger, and can then change output air temperature. This control response time delay must be considered carefully.
Next, the amount of valve response commanded by the controller in response to a temperature change at the air outlet will also affect the time it takes to achieve enough of a correction to meet needs. Thus, the P,I, and D constants in the controller will also have an effect on the dynamic response time.
Finally, you can not simply change the speed of the steam in the exchanger as the increased heat rate would increase outlet air temperature and the controller would then sense the higher temperature and close down the steam valve lowering steam velocity back toward the original level. To change the steam velocity, you would have to redesign the exchanger for lower internal cross sectional area. If you did that you may have to change PID constants in the control loop to achieve a critically damped control response.
You can not consider this as a simple heat transfer problem as there is an active feedback controller in the system.