How long does it take to transfer photos from MacBook Pro to iPhone?

Syncing from the Mac Photos library to iOS/iPadOS devices via USB has been historically flaky and sometimes very slow.

If all else fails my solution to that has been to backup the library, then open its package and delete the "iPod Photo Cache" folder:

On the Mac quit Photos and backup the Photos library (just in case because messing inside Photos library is generally not recommended because it might corrupt it).

Right click or control-click the "Photos Library.photoslibrary", choose "Show Package Contents" and delete the "iPod Photo Cache" folder (it holds a database of previous syncs and might have been corrupted. It is re-created in the next step). Empty trash and restart the computer.

Then try again.