Why does it takes forever to back up my iPhone using iCloud?

You need a stable internet connection and enough storage space in your iCloud, if you’re using iCloud backup. You can check how much storage you have by Settings>(Your Name)>iCloud>Manage Storage. To check how much storage you device back up is go Manage Storage>Backups>(Your device). I would suggest you use WiFi not data, and make sure your connection is stable.