turdchaser
You have two problems. For tub and shower you need to replace the cartridge in the valve and flush the lines before putting the new one in. The toilet is a seperate problem supply water pressure does not effect the flush of a toilet it only effects how long it would take for the tank to fill with water. The flush is effected by three things, the amount of water in the tank, how quickly the water gets from the tank to the bowl and if there is any obstruction in the discharge. The reason pressure does not matter is that when you flush the toilet the water in the tank is transferring to the bowl the water in the supply pipe is filling the tank back up. I reccommend taking a five gallon bucket of water and dumping it down the bowl if it goes fast the discharge is fine and the problem is the water transfer from the tank to the bowl. For water transfer problem make sure the water level is up to 1 inch below the fill tube in the tank, make sure water is filling the bowl when it is filling the tank thru the fill tube and finally take a small Allen wrench and probe in all the holes under the rim to remove calcium and mineral buildup. If you have a professional do this have them try what I mentioned before simply reccommending to replace the toilet. I hope this has helped and let us know how it turns out. 
