why is my phone taking long to backup?

Be patient, the backup has to look at many months of data before it can even start to proceed. It may still be a few hours before you really see much progress.

If you are using Wi-Fi, move the phone as close to the Wi-Fi router as possible and avoid using the phone as much as possible. Check tomorrow morning to see if things have completed.

Once the backup completes, subsequent backups will only take a few minutes in most cases unless you have added a lot of new data since the last backup.

How long is long? If this is the first backup, it's going to take some time, especially if you are using a slow Wi-Fi network.

My iphone 14 Pro Max has been taking more than 9 hours to backup to Mac and still not finish..

No idea how many hours left to complete

Any subsequent backup never take shorter than 1 hour

finally 23 hours!