Sometimes it also happens because the battery is deeply discharged, look new battery often doesn't have the chemical reaction going in place which is usually observed with a week old battery, I would say try using it for a week and and see if things speed up ! I have encountered the same issue with new battery put over my device but after a week the problem vanished. I think you are encountering the same try doing that and if you still didn't noticed any improvement then I would say you can always ask for a replacement for this one tooo.
