There are several possible sources of constriction.
I don't know what kind of water control system you have in your toilet, but the most common culprit in older toilets (to the best of my recollect) is the ball valve in the top of the filling tube. It is not uncommon for a foreign body to get lodged between the top of the ball and the top of the fill tube assemly, which doesn't allow the valve to fully open. There are usually two screws at the top of the fill tube assembly that you can take off and then remove the cap and see the assmbly and clean it out (be sure to turn your water off first :-)
I also always check the service line shut-off valve. I undo the tube from the bottom of the toilet tank (after shutting of the valve and flushing the water out of the tank) and put the end of the tube into an empty bucket and turn the valve on. You should get a hard stream of water. Sometimes the rubber washers will come lose in the shut-off valve and won't allow full water pressure.