New brake pads, very slow to stop
There are a few things to do when your brake pads aren't stopping your bike well:
-It could just be that they're new and haven't worn in yet, rub some sandpaper against the braking surface of the pad to remove the surface layer. Then use something to clean it to get off the lose rubber and grease from your hand. This can also happen on old pads with plenty of surface left, the pad can "glaze over". This hard layer can be removed in the same way.
-Your rim could have oil or grease on it. Clean the rim and the pads with hot soapy water and leave to dry naturally. 

If the pads were very cheap you could just take them back if these methods don't seem like they'll work. You might as well try the washing one because you're bit altering anything and they would still take the pads back.