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.
May be build up in the valve itself or the shower head.
Your water pressure is low. Consequently, when cold water is diverted to another application such as a toilet or a sink, the amount of cold water available at the shower mixing valve decreases as the low pressure is unable to keep up with the shower's demand. This causes a hotter mixture at the shower head. The solution is to either improve the cold water supply to the shower or to replace the shower control valve with a temperature compensating type. Pressure balancing is the cheapest and simply changes the shower temperature according to the pressure in the cold and hot water lines. When the toilet or sink is used, reduced pressure to the shower valve is detected and the valve reduces the hot water pressure, thereby maintaining the temperature. The more sophisticated approach is a thermostatic shower control valve. These are significantly more expensive, but will not reduce the pressure at the shower head (a drawback of the pressure balancing valve). The thermostatic valve would be required when one shower control valve is operating several shower heads. Moen, Kohler and Delta all make both types of control valves. They can be readily obtained from a local plumbing supply house.
Open up the actual shower head and remove the factory fitted restrictor.
Yes, you can install a tee in the riser. The pressure loss will be minimal since both shower heads will have a flow restrictor unless you remove them. If you remove them, you probably wont notice any difference. If you do notice a difference, put the flow restrictor back in the closest shower head to the shower valve.
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.
you have trash in your line take the shower head off and remove trash or possibly in water valve at shower
Seats in the main valve body are leaking by, allowing water to go to the shower head.
The valve in the shower control needs to be renewed.
By measuring each hole in the shower head and getting the pressure and then mathematically it can be figured out
There is a screen and/or a water saver in the shower head. The screen may be plugged with sediment from in the water.
It's usually due to a combination of two things: back pressure from the shower head, aggravated by the newer low-flow shower heads, acting against a slight leak at the diverter valve. I find that it gets worse after I've been away for a few days and assume that there's scale build-up in both places... the shower head and the diverter. You can try using a descaling solution on the shower head and/or checking that the diverter valve is seating properly and possibly greasing its seat with some silicone grease.
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.