Empty tank and remove the unit where the water comes in. Now look inside the bottom of the unit. Is there a rubber piece in there? Get a pair of pliars and pull it out.It's restricting the re-fill flow. I had the same problem and did this to two of my toilets and now they re-fill in just seconds. Don't know why they even put those in there.

Replace the washer in the Inlet Valve under the big nut, undo the the nut (make sure water supply is isolated first & flush pan) and remove the washer

it is a Diaphragm Washer for the Siamp Float Valve made by Wirquin, think the web address is www.wirquin.com. You can get one from b & q, make sure you put the washer the right way up and buy the right washer otherwise it will have the opposite affect it will not stop filling up