IPhone Internet & app downloading slow?

I recently just got my new iPhone the last one I got was stolen but the Internet on it was at a standard speed and apps would download in less then a minuets usually using it's own router NOT a wireless connection but now Internet connection is slot slower and apps will not download sometimes I go to bed and wake up 8 hours later and the app download wouldn't of even moved a bit is there any reason this is happening ??? Please help