1. Response time is already great, You have a reservoir of instantly available energy in the form of steam pressure ready to go whenever you need it. look up steam engines slipping their wheels, in fact one big problem with steam is that it is much easier to apply too much energy. Same problem with electric. they need a good torque management system to deliver only the power that is needed.
2. The reservoir response time is too long, Probably not a problem, that is the whole point of having a reservoir, but if it is, make it faster, fire tube boilers, more surface area, etc, a solvable problem. Consider the response time of the reservoir of an electric car. it is far worse than any steam engine, yet you say it has a faster response time.
No the real reason we don't use steam cars is that the engines are more expensive than internal combustion engines without all that much benefit.