Usually cases of a slowly decreasing amount of water are that a filter has become clogged. Does your water supply have sand particles coming through? Check the diverter heads for other faucets in the house. Are they getting many small particles trapped in them? Sand in one faucet tells you that all fixtures are getting clogged, some more slowly than others. This sandy junk can come through municipal water systems, or it can come from your own well if you are on one.
If you are getting much of this coming through, you may choose to install a whole house water filter to prevent that.
Whatever you find in those other faucets, if the problem is both the shower head and the tub spout, then the problem is in the fixture itself. Is this one of the new style fixtures that regulates the temperature of the water? I have read that those temperature regulators can sometimes be a problem themselves, improperly shutting down water flow.
If not that, assuming this is a cartridge based fixture, then you will need to pull the cartridge from the faucet. This entails removing the handle, then removing a locking nut that holds it together. Please shut off the water BEFORE you do any of this! The cartridge will come out, although this sometimes takes some amount of persuasion.
You can buy either a complete cartridge replacement from most manufacturers, or just a set of new o-rings etc., to rebuild the cartridge. Large home centers will have an entire wall filled with various such replacements. Bring in the old cartridge if possible to get a match. Knowing what company made your fixture will help.
A possibility is that a piece of rubber from an o-ring or other seal has cracked and become lodged in an orifice inside the cartridge. Look for this, or for other problems when you open up the cartridge.
Does the pressure return to normal if you reduce the output water temperature? Some electric showers will regulate the amount of water flowing through them to allow the water to reach the set temperature; at higher set temperatures, the flow rate is reduced so that the water remains in the heater longer. Given that your symptoms appeared recently, it could be that a heating element failed.
Best Answer
Take a gallon bucket and measure how long it takes to fill the bucket from the tub faucet, the shower head, and the shower pipe without the head on it. If the time to fill is relatively close between filling with the tub faucet and the headless shower, then it is probably the shower head. The shower head you buy must not allow more than 2.5 gallons per minute in the US, many are below 2.0 GPM. There are ways around it, such as modifying your shower head or putting in more than one shower head.