If you have galvanized steel pipes in the house, those could also restrict the flow. Eventually, depending on water conditions and the pipe, they all generally rut from the inside, and the interior diameter could be almost nothing. It could be the shutoff, the hose, or the fill valve, too. As to flush, if the small holes around the rim are clogged up with minerals and do not shoot water out, that can also affect the flush. A coathanger can sometimes punch them free again. But, it's probably just a less than optimal toilet. Lots of people end up with something caught in the trapway of the toilet, too. A pen, pencil, comb, toy, etc. can still allow the thing to flush, but occasionally clog or slow things down.