In our experience, tender times can vary substantially from port to port and cruise to cruise. A general rule of thumb: When the tenders are operated by the port (i.e. Santorini), things go very smoothly since they are so used to doing it. When the ship handles the tenders, it seems to come down to the experience of the crew. On our last cruise aboard the Epic, tendering in Patmos was a nightmare (disorganized, lengthy delays etc) which we attributed to the fact that the island is not a frequent destination.
Smoothest tendering experience ever was aboard the Norway which carried its own tenders that could each handle 500 passengers... but now I'm dating myself. 😉