Importing photos from iPhone to Photos app should list the new photos quickly. When the macOS Photos app shows "Preparing to import photos from iPhone" for a long time, it's basically stuck. The fix here is to toggle Airplane Mode and then back while connected to the mac. And then the photos got listed in the import window. But the phone was disconnected in between. So I plugged in to import the rest and this method wasn't working. I also tried with resetting the network settings on the phone, but it did not work. Then I restarted the mac and the photos showed in the import window. I faced this issue with iPhone 7 Plus. I did not get this issue with newer phones. The non-deterministic nature of software.