A Question related to the method of find lower bound : Trivial lower bounds
The trivial bound assumes that the program has to read the entire input. This is usually, but not always, the case. Similarly, it assumes that the program has to write each output item separately. This is usually, but not always, the case.

Since the program has to read the entire input, it has to do some operation for each input item. Similarly, it has to so some operation for each output item. This gives you the trivial bound, in terms of the number of basic operations.

Since we are usually only interested in complexity up to constant factors (big O complexity), the actual time that a basic operation takes doesn't enter the picture at all.