2.5 gpm is low flow. Most people complain about low flow rain type shower heads as it is. Reducing the flow any more would be called a mist type shower.
Back before low flow shower heads were available, I made a few flow restrictors using dimes. Drill a tiny hole through the center of the dime and install it between the shower head and the shower pipe. You can make it as restricted as you want depending on the size of the hole. Careful though, if you screw up, you're out a dime.