Why does backup take 14 hours on ipad pro?

The time required to perform a backup will be determined by many factors - including but not limited to:

• the quantity of data requiring backup
• the backup method used
• for iCloud backup, the available bandwidth of the WiFi network/internet connection

Larger backups, or poor network connection bandwidth, will result in extended backup times.

GilbertGiddyup wrote:

Why does ipad pro backup take 14 hours or more?

Because this is the first time you're backing up to icloud, it will take a long time if not longer.