Low shower head flow from Delta R1824 shower
Try exchanging the Delta head with one of the other shower heads you are happy with if you haven't already done so. No change in the flow means it's not in the head, and change means it is.
If it is in the head, since you are on a well and not a municiple water supply, you might consider removing the factory install flow controll device located just below the threaded area that mounts to the shower arm. These inserts can be a real bugger on some heads and each one is a little different than the last. The trusty nail often works well.
Your body sprays, I believe, are factory fixed at something like 2.2 gpm and non adjustable.
Also consider for a moment a garden hose with trigger spray head on it. When you squeeze the trigger the water will shoot out a certain maximum distance. If you were to take an ice pick and poke a hole in the hose, that maximum distance will be slightly reduced. The more holes in the hose the less the maximim distance. Correspondingly, the more holes you poke the less distance each hole will squirt because you have the same pressure and volume of water but more exit points.
A monitor has a flow rate of about 6 gpm at 85 psi which is pretty average for shower valves. If your other showers have only one exit point(i.e. one head) and the R1824 has 3 with the same pressure and volume, the garden hose illustration can also explain the volume change as well. Read More At : Delta...