I just installed a Delta shower massage head, and it has about half the water pressure of the old head I replaced. Is there any way to increase the water flow, like some kind of restricter in the hose or head?

Remove the shower head and you will see a rubber washer with holes around the edge. This would be a water saver. Pry it out and toss it. You may now rub-a-dub under a full stream. Cheers, Tom