We just had our bathroom remodeled and the new tub now drains very slowly and bubbles come up when its draining. Did the contractor not vent it properly?

If the trap were gurgling then you would have a vent problem. Bubbles indicate a partial blockage on down the line. What happens is that the discharge goes down the drain and hits a partial clog. It bounces back a bit sending air ahead of it. Those are the bubbles you see. This situation can only get worse so call him back to see if something was dropped down the pipe during installation. Now chances are he's not going to like being called back but you have a warranty on the work so stick by your guns and make him make it right. Good luck and let me know what happens. Tom