The time required to complete setup will be determined by many factors, including but not limited to:
- how much data must be restored to the new device from iCloud
- the speed/throughput of your internet connection
- the speed/throughput of your WiFi network
- Apple's back-end server loading; Apple’s regional servers may be busy
- whether or not the device is set-aside, or being actively used while background processes are completed
The only aspect that you may be able to directly influence is your WiFi network. You should note, however, that the slowest item in the chain will ultimately be the limiting factor.
Ignoring factors over which you have control, allowing background processes to complete without using the device will keep the time required to a minimum. Keep the device connected to its Power Adapter throughout the process - as this will keep the WiFi connection active while the device is not being used. If the device is set-aside without being connected to external power, the device will sleep - delaying completion of background tasks.