Answer Time performance for different select query types

would like to understand why some queries do not perform the way we expected regarding time response. 

We have performed the "SELECT queries" when the Event Table had a different number of rows in it, from 0 rows to more than 3 Billions (American Billions) along the weekend and we have plotted the answer times against the number of rows present.

In some of them we would expect a O(1) behavior while the response is O(n). There are also cases that the opposite happens (O(1) behavior when the expected we though should be O(n)).