Its typically one or more of three reasons: your end, the other end, or something in between. Either your connection is slow, the server you're downloading from has a slow connection, or the route between you and the server is having issues.What kind of Internet connection do you have? What upload/download speeds does your plan advertise? What speeds are you getting?
It could take long to download anything on your pc if it does not have enough memory space. It could also be caused opening many active windows at the same time and if your computer has virus. You need to do some routine cleaning and check up for your computer.
It could take long because your internet connection is slow, or because you have a anti-virus program that's slow in checking incoming data. Or perhaps there is a setting for bandwidth set to low on your computer. But often it's the first, the providers promise more than they can keep. Depends what type of connection you have and where you live.