There is a screen and/or a water saver in the shower head. The screen may be plugged with sediment from in the water.
You would use it to deliver a constant pressure at the shower-head, in situations where the water pressure is either low, or fluctuates.
By measuring each hole in the shower head and getting the pressure and then mathematically it can be figured out
The force of the spray from a shower head has more to do with the water pressure delivered by your plumbing. Some shower heads do have a valve that allows pressure to build in the shower head, thus delivering a more powerful spray, but sufficient water pressure would be necessary from the source. Visit the home improvement store and talk about the various options with a knowledgeable salesperson.
Aerator in shower head is plugged up. Unscrew head, clean it out, and replace. Make sure you have teflon tape on hand for the treads on the shower arm when putting the head back on.
A dirty or clogged screen can cause the low water presser remove shower head and clean. If not it could be calcium deposits built up on shower head if so soak shower head in a cleaning solution for several hours if that dose not work buy a new shower head. (note some shower heads have a low flow re-stricter in some cases you can remove this and get more water pressure)
The velocity does not depend on ther shower head, however it does depend on the cold mains pressure, a higher pressure will result in a higher velocity, and visa versa.
The simplest and lowest cost solution is to replace the shower head with one designed to increase the pressure.
Remove the shower head on the shower and see if it is plugged. Run the water with the shower head off and see if the pressure is better. Depending on where you live there may be a water saving feature on the shower head (mandated by law) for water consevation. Low flow shower heads often have a restricter in them. Delta has a brass washer with a very tiny hole in it and Moen has a rubber plug with 3 small holes in it.
you have trash in your line take the shower head off and remove trash or possibly in water valve at shower
A shower head with different pressure boost options that you can easily switch between and go from massage feel to shampoo rinsing.
you did not pay bill
The piping has to be the exact same length for each shower head from the tee where the cold and hot water feeds from the top of the diverter faucet.