This has happened before, but not for a long time... Now, in AC15, any simply COPY (does not matter how many objects)... takes 2 minutes or more to give me back the mouse...
Same problem here. Had the problem in AC14 before installing 15. Once I installed AC15 it was normal again, no problem with copy/paste, but it has recently begun acting like it did in AC14 again where it takes a very long time to copy. The PC just hangs for a bit until it finishes the copy command. I haven't been able to make heads or tails of it since I don't recall anything changing in either AC14 or AC15 when it began.
AC 6-29 - Ryzen9-9950X / 96GB RAM / RTX 5090 / Win 11 Pro
As I recall there was a workaround of setting the Save as... > Project Archive to not include linked textures (strange but it worked). Perhaps you can try that.