Our 2nd floor bathroom sink has started to run quite slow. The tub and toilet and all the other water outlets in the house are fine. Why is the sink running so slow?

Why is the sink running so slow? Could be several things, Check down the drain for hair build up first. If it looks clear then I'd suspect a clogged drain line in the wall,
Remove the traps "J" bend and send a snake up into the wall, (see image). Put out about 7 feet of snake and then flush with hot water. Good luck, Tom