Why does a new sink start to drain so slow?

Since it is a new construction, call the contractor/plumber back. It should not do that--that is not proper. They may have gunked it up with adhesive, or they may have rinsed construction materials in it that is stopping it up. Have you disposed of any item that may be caught in the trap? Something is wrong with it.

Hi There

Possibly the Air is getting trapped, check the breather pipe and vent holes for obstructions, also ensure that the pipe work is of same size through out the plumbed section, i mean one size.

Also check whether there are any other items connected to the source drain pipework IE: Your washing machine/dish washer sometimes these can cause Air locks, if this is the case remove the drain pipe for these appliances one at a time and see if it cures the problem.

Hope it helps

Ash

Air lock. The waterway is fine as it runs once you have broken the lock. Look for the vent and then unblock it. For water to go down, air usually has to go up.

Since its a new construction, perhaps the builder forgot to install a vent !

WAS THE SINK REPLACEING AN EXISTING SINK? OR IS IT A COMPLETE NEW INSTALL IF NEW INSTALL IT MIGHT NOT HAVE THE PROPER VENTING ON THE DRAIN PIPE.

ther is a blocked pipe some wear call whover instaled yor sink they might have to run a snake though the pipes.

the drain trap gasket is too large and is blocking the drain holes, or something is blocking the vent stack.

perhaps grease stuck in the drain

not properly vented