What causes the iPad to show
Bookmarks are being synced. Please add the bookmarks once syncing has completed
when attempting to add a bookmark or favorite in Safari and how can it be prevented?
Your iPad or any iOS/macOS device logged into an iCloud with Safari sharing in
Settings -> iCloud -> Safari will sync history, bookmarks, and logins. Any changes to the bookmarks will trigger the sync, forcing you to wait. Its a pain in the arse to manage bookmarks with this always popping up.
Disable the syncing in your iCloud settings under Safari.
It's part of iCloud! You can either turn off iCloud bookmark sync
I am not sure it can be done for individual bookmarks, I believe it is a system wide setting.