There is no simple answer and there could be many reasons. Here are a few.
Security checks that are doing more than just creating accounts such as checking known blacklists, spoofing checks, calculating exact location of signals/account, etc…
The software may not be optimized, buggy, bottle-necked, etc…
The cloud providers is causing delays either knowingly or unknowingly.
Something else limiting the capacity such as database transactions, topology, no federations, etc….