Kitchen sink was not draining . Removed p trap and cleaned it . Replaced but when sink filled with water it was draining very slowly . What is wrong ?

You have a blockage beyond the trap. Snake the sink drain line. Put out enough snake to reach the main drain.

I agree with hkstroud, You can remove the "J" bend from the trap and snake the drain line, (see images). You will run into a 90 degree bend about 8 inches in but once around that you only have to put out 6 more feet of cable. After, flush with hot water.

Agree with both gentlemen, once you go inside the wall there is a 90 degree bend so really work that snake around, twist and push several times and it will finally go in another few feet.

Once you clear the blocked drain, start paying attention what you put in the sink - and if you have garbage disposer - than what you put inside garbabe disposer, as well. Always use lots of hot water to flush grease or grinded food from garbage disposer. If possible, scoop food off the plate into garbage bag before you put it in the sink. That way, you will prevent future drain back ups

Milo