Why does it take a long time to assign a new folder for Photos? It is taking hours and hours for me.

Please explain more? What are you trying to do? I am not sure, what you are referring to by "New folder for Photos"?

Are you trying to create folders in your Photos Library? Trying to move the Photos Library? Trying to select a new System Photo Library?

To make a library the System Photo Library, open it in Photos, open the Preferences, and click the "Use as System Library" button.