An initial iCloud backup will take considerably longer than subsequent incremental updates to an existing backup.
The time required to perform the iCloud backup will be determined by many factors, including but not limited to:
- how much data must be backed-up to iCloud
- the speed/throughput of your internet connection
- the speed/throughput of your WiFi network
- server loading; assuming an iCloud backup, Apple’s regional servers may be busy
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.