What temp do you run the boiler? Typically, when running a priority zone for the WH, it fires the boiler to max temp to quickly recover (rather than some lower temp that might be used for space heating). If you aren't setup to get high water temp, that would account for the slower recovery along with the smaller piping. Also, if you run a separate circulator, you can specify one specified by the WH for max flow, to minimize the time it takes. If the house gets appreciably cooler while reheating the tank, maybe it's time to think about some more insulation.