Unscrew the showerhead from the pipe. Examine the inside of the pipe, and the inside of the showerhead, for calcium deposits. Soak a rag or sponge in calcium deposit cleanser and insert it into the pipe to clean it.
Insert a cleaning brush into the pipe to knock loose any stubborn calcium deposits. Insert the brush, then pull it out. Repeat the process until no more deposits come out.
Adjust the temperature settings on your hot water heater if the water coming out of your Delta showerhead goes cold after just a few minutes. Experiment with different temperature settings until you have sufficient hot water for your shower.