With very cold water from the hookup, I have had the best luck with shutting off the cold water valve that controls the shower supply. This is in the wet bay. Once that valve is closed, only water which passes through the heater will go to the shower. Otherwise, the shower mixer valve will probably let some unheated water into the shower head. If that water is 40 degrees, you'll notice it!!
As a corollary, if I have that valve closed in Florida, where the water coming out of the tap is already about 80, I practically burn myself in the shower!!
Yes, the overall system is more difficult to manage, but I get to take hot showers for as long as I want!