On airliners, the safety demonstration includes many items (seat belt, life vest, evacuation, etc.) and each of those items may raise questions from the passengers (when to inflating the life vest, how to unlocking the seat belt, ...) and thus need more time to explain why those items are detailed.
I haven't heard of any regulations about the length of the safety demo, just the content, which is pretty much as you described. However it's in the airline's interest to keep it as short as possible. The aircraft cannot take-off before the chief of cabin notifies the cockpit that the cabin is ready for take-off. The cabin is ready after all passengers are on their seats, safety demo completed, galleys secured and hand luggage stowed away.