If your toilet cistern is taking too long to refill and be ready for the next user (Mine was taking 10 minutes!) you may have found some of the great articles and videos that basically tell you to dismantle the float valve and clean out the blockage. A few others suggest you may have low water pressure but are using a high pressure nozzle.
Try those approached first and if one of them solves your problem, that’s great and you can stop reading now.
Another cause can be that the rubber diaphragm in the float valve is the wrong way round.
Here’s the story…
I actually had two problems with my cistern. As well as being slow to fill, after about 18 hours, it started to overflow very gently. I replaced the rubber diaphragm being careful to put it the same way round as the old one. This fixed the overflow – although adjusting this correctly was difficult because it took tens of hours for the water flow to reduce from extremely low to zero. However, the filling was just as slow as previously. Watching carefully, I noticed that the flow started off pretty feeble and rapidly declined as the cistern filled. Twelve hours later, it was still filling at about one drop per minute.
So I partially dismantled it again and examined the diaphragm and nozzle interaction. As the float went up, the diaphragm quickly made gentle contact with the nozzle but real water-stopping contact needed the float past the top. This caused the flow to slow down almost immediately but never really stop.
So I put the diaphragm in the other way.
Now, when the cistern was empty, the diaphragm was quite a long way from the nozzle so water rushed in and the toilet was ready for the next user in 30 seconds. When the cistern was nearly full, the diaphragm and nozzle met and the diaphragm was rock hard so stopping the flow quickly.
The diaphragm design seems to vary a bit so this video shows to hold and test a diaphragm. First, my thumbs are pressing the soft side and then the hard side. You want the nozzle facing the hard side
It's just possible that none of this article is making sense to you because you've got a different sort of "ball cock" or float valve.. Here's a view of the inside on my cistern...
.. and here's a closer view of the float valve
Some of you will be have at simple small black disc-shaped "washer" which is the same on both sides rather than the larger, more complicated diaphragm. You've got a different sort of float valve and this article doesn't apply - Sorry!