Efficiency/Redundancy in Chomsky normal form

Question

I have a context-free grammar with the following production rules, $S$ being the start symbol:

$$\begin{align*} S &\to AB \\ A &\to a \\ B &\to a\end{align*}$$

Is this in Chomsky normal form?

My problem is I thought CNF is supposed to be an efficient way to write a grammar, yet the grammar is not efficient in the sense that we can clean its rules as follows:

$$\begin{align*}S &\to AA\\ A &\to a\end{align*}$$