Why does it takes to long when updating/restore iphone 3gs to 6.1?

I have an iphone 3gs i'm trying to restore/update my iphone to the latest ios 6.1
my problem is that when is i'm trying to update/restore it to ios 6.1 the process take to long like 2hours or more… Its stuck on the line thats say's restoring iphone software and the progress bar stays on the middle… Please can anyone answer it as soon as possible… Thank you

Added (1). There's nothing on my iphone it is new restored to 4.3.5

The 3gs is an older model, and likely has trouble handling the new operating system (the 3g, for example, is capped at 4.2.1). I would suggest downgrading to a lower version. Also it might help to clear up applications, extra photos, and things you don't use, so as to have more space.

Another thing, try doing a hard reset by holding down the home and power buttons simultaneously until the screen flashes white and goes black. Then keep holding until the apple symbol comes back on, and release. This will clear up the cache, etc that might be slowing the phone down.

And while you're at it, why not jailbreak? XD. There are a lot of nice features through jailbreaking, such as the ability to make a customized os, or set the amount of memory to be reserved for system.