Toilet refills slowly and is loud

I need help with a toilet that refills really slowly and loudly (water noise). It hasn't always been this way, but this toilet has always been a weak flusher. My husband says he thinks that the toilet isn't vented well, but he knows as much as I do about toilets or plumbing - which is nothing. What can I do to find the problem??

Is the angle stop valve supplying the toilet all the way open?
If it is then a new fill valve may be needed.

Chances are venting has little to do with a weak flush...
What is the manufacturer and model of the toilet and its age?
That may have a lot more to do with the weak flush....

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.

If you do decide to replace the fill valve, then replace the supply line also, use a 20 inch supply line and loop it in such a way that it will not kink.