Photos library very slow to update

When I take photos on my iPhone, often they appear pretty quickly on my Mac Studio desktop and MacBook Air, but sometimes, like right now, the Photos library is not updating even after 4 hours of waiting. The new photos are on iCloud online, so they have uploaded, but not on my Mac Studio, however they are on my MacBook. Why is my Mac Studio not updating? It says it is updating the entire library!! Why?

Incidentally, the library is optimized on my MacBook, but originals on my Mac Studio.

[Edited by Moderator]