Having worked at the Apple Store as a Genius (I despise this term to describe a retail employee), more times than not, when someone had a library that large, iPhoto wasn't efficient, but Aperture was. I haven't worked at the store for over 4 years, but I do have clients who are photographers and NONE of them use Photos for any organization or editing. Photos, especially with iCloud Photo Library turned on, isn't efficient in anyway with handling large libraries. My sister has a library that well over 300GB and complains about Photos constantly. It simply isn't meant to manage a library that large. Its very inefficient at doing so...I've tried using said Photos Library on a 2018 15" with i9/32GB/4TB and guess what, still inefficient. It's not a horsepower problem, it's the coding of Photos. It EATS up RAM!