It's memory and disk paging both.

There's also a hot spot problem with databases. That's the performance problem with autoincrement integers. If you are always writing to the same page on disk, then every write has to lock the same page.

Uuidv7 is a trade off between a messy b-tree (page splits) and a write page hot spot (latch contention). It's always on the right side of the b-tree, but it's spread out more to avoid hot spots.

That still doesn't mean you should always use v7. It does reversibly encode a timestamp, and it could be used to determine the rate that ids are generated (analogous to the German tank problem). If the uuidv7 is monotonic, then it's worse for this issue.