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)).